MainActivity --> Instructions or AdminLogin --> UserLogin or RegisterCandidate

E/AndroidRuntime: FATAL EXCEPTION: main
              java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.palakjain.onlinevotingschool/com.example.palakjain.onlinevotingschool.AdminLogin}: java.lang.NullPointerException
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2360)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2412)
                  at android.app.ActivityThread.access$600(ActivityThread.java:162)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1364)
                  at android.os.Handler.dispatchMessage(Handler.java:107)
                  at android.os.Looper.loop(Looper.java:194)
                  at android.app.ActivityThread.main(ActivityThread.java:5388)
                  at java.lang.reflect.Method.invokeNative(Native Method)
                  at java.lang.reflect.Method.invoke(Method.java:525)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
                  at dalvik.system.NativeStart.main(Native Method)
               Caused by: java.lang.NullPointerException
                  at com.example.suniljain.onlinevotingschool.AdminLogin.onCreate(AdminLogin.java:39)
                  at android.app.Activity.performCreate(Activity.java:5141)
                  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1084)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2307)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2412)
                  at android.app.ActivityThread.access$600(ActivityThread.java:162)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1364)
                  at android.os.Handler.dispatchMessage(Handler.java:107)
                  at android.os.Looper.loop(Looper.java:194)
                  at android.app.ActivityThread.main(ActivityThread.java:5388)
                  at java.lang.reflect.Method.invokeNative(Native Method)
                  at java.lang.reflect.Method.invoke(Method.java:525)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:833)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:600)
                  at dalvik.system.NativeStart.main(Native Method)

public class MainActivity extends AppCompatActivity {

Button btnConductVoting, btnInstructions, btnSkipInstructions;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    btnConductVoting = (Button) findViewById(R.id.btnConductVoting);
    btnInstructions = (Button) findViewById(R.id.btnInstructions);
    btnSkipInstructions = (Button) findViewById(R.id.btnSkipInstructions);

    btnConductVoting.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(MainActivity.this, AdminLogin.class);
            startActivity(i);
        }
    });

    btnInstructions.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(MainActivity.this, Instructions.class);
            startActivity(i);
        }
    });

    btnSkipInstructions.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(MainActivity.this, UserLogin.class);
            startActivity(i);
        }
    });

}

public class Instructions extends AppCompatActivity{
Button btnStartVoting;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    btnStartVoting = (Button) findViewById(R.id.btnStartVoting);

    btnStartVoting.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(Instructions.this, UserLogin.class);
            startActivity(i);
        }
    });
}

public class AdminLogin extends AppCompatActivity{

Button btnAdminLogin;
EditText etUsername;
EditText etPassword;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    btnAdminLogin = (Button) findViewById(R.id.btnAdminLogin);
    etUsername = (EditText) findViewById(R.id.etAdminPassword);
    etPassword = (EditText) findViewById(R.id.etAdminPassword);

    btnAdminLogin.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if(etUsername.getText().toString().equals("administrator") && etPassword.getText().toString().equals("password")){
                Intent i = new Intent(AdminLogin.this, RegisterCandidate.class);
                startActivity(i);
            }
        }
    });

}

public class UserLogin extends AppCompatActivity{

//TextView tvUserForgotPassword;
TextView tvNewUserSignUp;
//Button btnUserLogIn;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    tvNewUserSignUp = (TextView) findViewById(R.id.tvNewUserSignUp);

    tvNewUserSignUp.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent i = new Intent(UserLogin.this, NewUser.class);
            startActivity(i);
        }
    });

}

public class RegisterCandidate extends AppCompatActivity{
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}

public class NewUser extends AppCompatActivity{
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}

@Override
   protected void onCreate(@Nullable Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   // set contentview here before finding views otherwise they will be null
    btnAdminLogin = (Button) findViewById(R.id.btnAdminLogin);
   }