@Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login);
        loginconfig = new AppConfig();

        txtHeader = (TextView) findViewById(R.id.login_header);
        editEmail = (EditText) findViewById(R.id.loginEmail);
        editPassword = (EditText) findViewById(R.id.loginPassword);
        txtSignUp = (TextView) findViewById(R.id.txtSign);
        btnLogin = (Button) findViewById(R.id.btnLogin);

        sp_session = this.getSharedPreferences("Session_User", MODE_PRIVATE);

        editor_session = sp_session.edit();

        if (sessionManager.isLoggedIn()) {

            String id = sp_session.getString("user_id", null);

            Intent intent = new Intent(getApplicationContext(), MainActivity.class);
            intent.putExtra("user_id", id);
            startActivity(intent);
            finish();
        }

        btnLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(getApplicationContext(), MainActivity.class);
                startActivity(intent);
            }
        });


        txtSignUp.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                if (loginconfig.validateBlank(getApplicationContext(), editEmail, "Email Address")) {
                    if (loginconfig.validateBlank(getApplicationContext(), editPassword, "Password")) {

                        new send_Login_Info().execute();
                    }
                }

            }
        });
    }


    public class send_Login_Info extends AsyncTask<String, String, JSONObject> {

        String lemail = editEmail.getText().toString();
        String lpassword = editPassword.getText().toString();

        JSONParser jsonParser = new JSONParser();

        private static final String LOGIN_URL = "http://laundry.cattpre.com/mob_app/index/user_login.php";

        private static final String TAG_MESSAGE = "result";


        @Override
        protected void onPreExecute() {
            super.onPreExecute();

            pDialog.show();
        }

        @Override
        protected JSONObject doInBackground(String... args) {
            try {

                HashMap<String, String> params = new HashMap<>();

                params.put("name", lemail);
                params.put("password", lpassword);

                Log.d("request", "starting");

                JSONObject json = jsonParser.makeHttpRequest(
                        LOGIN_URL, "POST", params);

                if (json != null) {
                    Log.d("JSON result", json.toString());

                    return json;
                }

            } catch (Exception e) {
                e.printStackTrace();
            }
            return null;
        }


        protected void onPostExecute(JSONObject json) {

            if (pDialog != null && pDialog.isShowing()) {
                pDialog.dismiss();
            }

            if (json != null) {
               /* Toast.makeText(SignUp.this, json.toString(),
                        Toast.LENGTH_LONG).show();
*/
                try {
                    //  result = json.getInt(TAG_SUCCESS);
                    result = json.getString(TAG_MESSAGE);
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            }

            if (result.equals("true")) {
                sessionManager.createLoginSession(editEmail.getText().toString(), editPassword.getText().toString());
                Toast.makeText(getApplicationContext(), "Login Successfull", Toast.LENGTH_SHORT).show();

                try {
                    userinfo = json.getJSONArray("users_info");


                    for (int i = 0; i < userinfo.length(); i++) {
                        JSONObject c = userinfo.optJSONObject(i);
                        if (c != null) {
                            user_id = c.getString("users_id");

                        }

                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }
                editor_session.putString("user_id", user_id);

                editor_session.commit();
                Intent i = new Intent(LoginActivity.this, MainActivity.class);
                i.putExtra("user_id", user_id);
                startActivity(i);
                finish();

            } else {
                Toast.makeText(getApplicationContext(), "Invalid Deatils... Try Again", Toast.LENGTH_SHORT).show();

            }
        }

    }

}

public class SessionManager
{
    SharedPreferences pref;

    // Editor for Shared preferences
    SharedPreferences.Editor editor;

    // Context
    Context _context;

    // Shared pref mode
    int PRIVATE_MODE = 0;
    private static final String PREF_NAME = "Laundry";

    // All Shared Preferences Keys
    private static final String IS_LOGIN = "IsLoggedIn";

    // User name (make variable public to access from outside)
    public static final String KEY_NAME = "name";

    // Email address (make variable public to access from outside)
    public static final String KEY_PASS = "password";

    // Constructor
    public SessionManager(Context context){
        this._context = context;
        pref = _context.getSharedPreferences(PREF_NAME, PRIVATE_MODE);
        editor = pref.edit();
    }
    /**
     * Create login session
     * */
    public void createLoginSession(String name, String password){
        // Storing login value as TRUE
        editor.putBoolean(IS_LOGIN, true);

        // Storing name in pref
        editor.putString(KEY_NAME, name);

        // Storing email in pref
        editor.putString(KEY_PASS, password);

        // commit changes
        editor.commit();
    }
    /**
     * Check login method wil check user login status
     * If false it will redirect user to login page
     * Else won't do anything
     * */
    public void checkLogin(){
        // Check login status
        if(!this.isLoggedIn()){
            // user is not logged in redirect him to Login Activity
            Intent i = new Intent(_context, LoginActivity.class);
            // Closing all the Activities
            i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

            // Add new Flag to start new Activity
            i.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

            // Staring Login Activity
            _context.startActivity(i);
        }

    }
    /**
     * Get stored session data
     * */
    public HashMap<String, String> getUserDetails(){
        HashMap<String, String> user = new HashMap<String, String>();
        // user name
        user.put(KEY_NAME, pref.getString(KEY_NAME, null));

        // user email id
        user.put(KEY_PASS, pref.getString(KEY_PASS, null));

        // return user
        return user;
    }

    /**
     * Clear session details
     * */
    public void logoutUser(){
        // Clearing all data from Shared Preferences
        editor.clear();
        editor.commit();

        // After logout redirect user to Loing Activity
        Intent i = new Intent(_context, LoginActivity.class);
        // Closing all the Activities
        i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);

        // Add new Flag to start new Activity
        i.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

        // Staring Login Activity
        _context.startActivity(i);

    }
    // Get Login State
    public boolean isLoggedIn(){
        return pref.getBoolean(IS_LOGIN, false);
    }
}

05-10 06:25:29.479 17037-17037/com.projects.cattpre.laundry E/AndroidRuntime: FATAL EXCEPTION: main
                                                                              Process: com.projects.cattpre.laundry, PID: 17037
                                                                              java.lang.RuntimeException: Unable to start activity ComponentInfo{com.projects.cattpre.laundry/com.projects.cattpre.laundry.LoginActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean com.projects.cattpre.laundry.SessionManager.isLoggedIn()' on a null object reference
                                                                                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
                                                                                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
                                                                                  at android.app.ActivityThread.access$800(ActivityThread.java:151)
                                                                                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
                                                                                  at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                                  at android.os.Looper.loop(Looper.java:135)
                                                                                  at android.app.ActivityThread.main(ActivityThread.java:5254)
                                                                                  at java.lang.reflect.Method.invoke(Native Method)
                                                                                  at java.lang.reflect.Method.invoke(Method.java:372)
                                                                                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
                                                                                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
                                                                               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean com.projects.cattpre.laundry.SessionManager.isLoggedIn()' on a null object reference
                                                                                  at com.projects.cattpre.laundry.LoginActivity.onCreate(LoginActivity.java:53)
                                                                                  at android.app.Activity.performCreate(Activity.java:5990)
                                                                                  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
                                                                                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
                                                                                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
                                                                                  at android.app.ActivityThread.access$800(ActivityThread.java:151)
                                                                                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
                                                                                  at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                                  at android.os.Looper.loop(Looper.java:135)
                                                                                  at android.app.ActivityThread.main(ActivityThread.java:5254)
                                                                                  at java.lang.reflect.Method.invoke(Native Method)
                                                                                  at java.lang.reflect.Method.invoke(Method.java:372)
                                                                                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
                                                                                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
05-10 06:30:29.814 17037-17037/com.projects.cattpre.laundry I/Process: Sending signal. PID: 17037 SIG: 9