package com.windroid.dinas;

.....

public class LoginActivity extends AppCompatActivity {

    GlobalVariables globalVariables;

    EditText miUser;
    EditText miPass;

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

        globalVariables = (GlobalVariables)getApplicationContext();

        miUser = (EditText) findViewById(R.id.eUser);
        miPass = (EditText) findViewById(R.id.ePass);
    }

    public void Login(View view) {
        new MiTurno().execute("http://10.0.3.2/baradm/sesion.php?id="+miUser.getText().toString());
    }

package com.windroid.dinas;

......

    public class MiTurno extends AsyncTask<String, Void, String> {

    GlobalVariables globalVariables;

    public GlobalVariables getGlobalVariables() {
        return globalVariables;
    }

    @Override
        protected String doInBackground(String... urls) {
            // params comes from the execute() call: params[0] is the url.
            try {
                return downloadUrl(urls[0]);
            } catch (IOException e) {
                return "Unable to retrieve web page. URL may be invalid.";
            }
        }

        // onPostExecute displays the results of the AsyncTask.
        @Override
        protected void onPostExecute(String result) {

            try {
                JSONArray ja = new JSONArray(result);

               if (!ja.getString(1).equals(miUser.getText().toString())) {
----> AQUI SE GENERA EL ERROR Y NO SE COMO SOLUCIONARLO
                    Toast toast = Toast.makeText(LoginActivity.this, "USUARIO NO EXISTE", Toast.LENGTH_LONG);
                    toast.setGravity(Gravity.CENTER_VERTICAL, 0, 0);
                    toast.show();
                }
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }