The title is still null when you are calling 
mAlertDialog dialog = new mAlertDialog();
dialog.setTheTitle(getActivity().getString(R.string.invalidTimeTitle));
so what you can do is
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.dialog_invalid_time, container, false);
        title = (TextView)view.findViewById(R.id.titleText);
        setTheTitle(getActivity().getString(R.string.invalidTimeTitle));
        return view;
    }
// call it
mAlertDialog dialog = new mAlertDialog();
dialog.show(fm, "InvalidTime");
2
solved Unexpected NullpointExpection inside a setter [duplicate]