Fix wxPython 4.1.1 assertion

[hasii2011]

Traceback (most recent call last):
File "/Users/humberto.a.sanchez.ii/PycharmProjects/PyUt/src/org/pyut/ui/AppFrame.py", line 559, in _OnMnuFileOpen
self._loadFile()
File "/Users/humberto.a.sanchez.ii/PycharmProjects/PyUt/src/org/pyut/ui/AppFrame.py", line 846, in _loadFile
self.updateCurrentDir(dlg.GetPath())
wx._core.wxAssertionError: C++ assertion ""!HasFlag(wxFD_MULTIPLE)"" failed at ../../../ext/wxWidgets/include/wx/filedlg.h(109) in GetPath(): When using wxFD_MULTIPLE, must call GetPaths() instead