Self Destructing Message error

        mLoginButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                String username = mUsername.getText().toString();
                String password = mPassword.getText().toString();

                username = username.trim();
                password = password.trim();

                if(username.isEmpty() || password.isEmpty()){
                    AlertDialog.Builder builder = new AlertDialog.Builder(LoginActivity.this);
                    builder.setMessage(R.string.login_error_message)
                    .setTitle(R.string.login_error_title)
                    .setPositiveButton(android.R.string.ok, null);
                    AlertDialog dialog = builder.create();
                    dialog.show();

                }
                else{
                    //login
                    ParseUser.logInInBackground(username, password, new LogInCallback() {

                        @Override
                        public void done(ParseUser user, ParseException e) {
                            if (e == null){
                                //Success!
                                Intent intent = new Intent(LoginActivity.this, MainActivity.class);
                                intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                                intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
                                startActivity(intent);
                            }
                            else{
                                AlertDialog.Builder builder = new AlertDialog.Builder(LoginActivity.this);
                                builder.setMessage(e.getMessage())
                                .setTitle(R.string.login_error_title)
                                .setPositiveButton(android.R.string.ok, null);
                                AlertDialog dialog = builder.create();
                                dialog.show();
                            }
                        }
                    });
                }
            }
        });