Null reference onPostExecute

Question

I would like the help of you, i try to declare "if (result!=null && result.contains("login_ok")) {", but the app answer wrong pass, same insert a real pass.

logcat error

03-31 09:16:51.509 20585-20585/estudo.br.loginphpmysql E/AndroidRuntime: FATAL EXCEPTION: main Process: estudo.br.loginphpmysql, PID: 20585
java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.contains(java.lang.CharSequence)' on a null object reference at estudo.br.loginphpmysql.Login$Askdata.onPostExecute(Login.java:85) at
estudo.br.loginphpmysql.Login$Askdata.onPostExecute(Login.java:72) at
android.os.AsyncTask.finish(AsyncTask.java:667) at android.os.AsyncTask.-wrap1(AsyncTask.java) at
android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:684) at
android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:154) at
android.app.ActivityThread.main(ActivityThread.java:6119) at java.lang.reflect.Method.invoke(Native Method) at
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) at
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) at

and the class code

public class Login extends AppCompatActivity {
        EditText txtEmailL, txtPassL;
        Button btnLogin; 
        String url = "";
        String parameters= "";

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.login_activity);    
            txtEmailL = (EditText) findViewById(R.id.txtEmailL);
            txtPassL = (EditText) findViewById(R.id.txtPassL);    
           btnLogin = (Button) findViewById(R.id.btnLogin);
        }    
        public void txtforgot(View view) {
            Intent intent = new Intent(Login.this, RedefinesPass.class);
            startActivity(intent);
        }

        public void txtregister(View view) {
            Intent intent = new Intent(Login.this, Register.class);
            startActivity(intent);
        }

        public void btnlogin(View view) {
            ConnectivityManager connMgr = (ConnectivityManager)
                    getSystemService(Context.CONNECTIVITY_SERVICE);
            NetworkInfo networkInfo = connMgr.getActiveNetworkInfo();    
            if (networkInfo != null && networkInfo.isConnected()) {
                String email = txtEmailL.getText().toString();
                String pass= txtPassL.getText().toString();

                if (email.isEmpty() || pass.isEmpty()) {
                    Toast.makeText(getApplicationContext(), "fill in fields!", Toast.LENGTH_LONG).show();
                } else {
                    parameters = "email= " + email + "&pass= " + pass
                    url = "http://localhost/logintest";
                    Askdata obj = new Askdata();
                    obj.execute(url);
                }
               } else {
                Toast.makeText(getApplicationContext(), "No internet connection", Toast.LENGTH_LONG).show();
            }     
            }   
    line 72       private class Askdata extends AsyncTask<String, Void, String> {    
            @Override
            protected String doInBackground(String... urls) {    
                return Connection.postDados(urls[0], parameters);
            }  
            @Override
            protected void onPostExecute(String result) {    
    line 85                    if (result.contains("login_ok")) {
                        Intent inicio = new Intent(Login.this, MainScreen.class);
                        startActivity(inicio);    
                    } else {
                        Toast.makeText(getApplicationContext(), "User or pass Wrong!", Toast.LENGTH_LONG).show();
                    }    
                }    
            }   
            @Override
        protected void onPause(){
            super.onPause();
            finish();
        }
    }