ID 509595 - Unable to build widgets derived from QT4FormWidgets.FilterablePopupFormWidget

Problem summary
When instantiating a class derived from QT4FormWidgets.FilterablePopupFormWidget Katana produces the following error:

[ERROR python.QT4FormWidgets.FormWidget]: Error building control widget: 'NoneType' object has no attribute 'aboutToShow'

 
Furthermore, calling the show() method on that object produces an additional error:

[ERROR python.root]: An AttributeError occurred in "FilterablePopupFormWidget.py": 'MyPopupForm' object has no attribute '_FilterablePopupFormWidget__popupButton'    Traceback (most recent call last):      File "FormWidget.py", line 1922, in showEvent      File "BaseValueFormWidget.py", line 140, in _thaw      File "BaseValueFormWidget.py", line 167, in valueChangedEvent      File "FilterablePopupFormWidget.py", line 134, in _updateControlWidget    AttributeError: 'MyPopupForm' object has no attribute '_FilterablePopupFormWidget__popupButton'

 
Customer reported version
Katana 5.0v2
 
Customer reported platform
RHEL 7
 
Steps to reproduce
 
1)  Run the following code in Katana's Python tab:
 

class MyPopupForm(QT4FormWidgets.FilterablePopupFormWidget):     pass policy = QT4FormWidgets.PythonValuePolicy('', {'foo': 'bar'}).getChildByName('foo') w = MyPopupForm(None, policy, FormMaster.KatanaWidgetFactory) w.show()

 
Expected behaviour
A new window is displayed with the form widget and no errors are produced.

 
Actual behaviour
A new window is displayed with the FilterablePopupFormWidget missing its button and errors are present in the console.

 
Workaround
Unknown.
 
Reproduced by support
This bug has been reproduced in:
 
Katana 5.0v1/2/3 - Windows 10
Katana 4.5v1/2/3 - Windows 10
 
Unable to reproduce bug in:
 
Katana 4.0v8 - Windows 10
 
Earliest version tested

Katana 4.0v8 - This issue doesn't appear in this version and has regressed