Ribbit keeps crashing upon "signing up". Not sure what causes this.

public class SignUpActivity extends Activity {

    protected EditText mUserName;
    protected EditText mPassword;
    protected EditText mEmail;
    protected Button mSignUpButton;

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

        requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
        setContentView(R.layout.activity_sign_up);

        mUserName = (EditText) findViewById(R.id.usernameField);
        mPassword = (EditText) findViewById(R.id.passwordField);
        mEmail = (EditText) findViewById(R.id.emailField);
        mSignUpButton = (Button) findViewById(R.id.signupButton);

        mSignUpButton.setOnClickListener(new View.OnClickListener() {

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

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

                if(username.isEmpty() || password.isEmpty() || email.isEmpty()){
                    AlertDialog.Builder builder = new AlertDialog.Builder(SignUpActivity.this);
                    builder.setMessage(R.string.signup_error_message)
                           .setTitle(R.string.signup_error_title)
                           .setPositiveButton(android.R.string.ok, null);
                    AlertDialog dialog = builder.create();
                    dialog.show();
                }
                else{

                    setProgressBarIndeterminateVisibility(true);
                    ParseUser newUser = new ParseUser();
                    newUser.setUsername(username);
                    newUser.setPassword(password);
                    newUser.setEmail(email);
                    newUser.signUpInBackground(new SignUpCallback() {

                        @Override
                        public void done(ParseException e) {
                            setProgressBarIndeterminateVisibility(false);
                            if(e == null){
                                // success
                                Intent intent = new Intent(SignUpActivity.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(SignUpActivity.this);
                                builder.setMessage(e.getMessage())
                                       .setTitle(R.string.signup_error_title)
                                       .setPositiveButton(android.R.string.ok, null);
                                AlertDialog dialog = builder.create();
                                dialog.show();
                            }
                        }
                    });
                }
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.sign_up, menu);
        return true;
    }

}```