Fix crash if selecting siblings of root widget

Traceback (most recent call last):
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.windows.treeeditwindow", line 1186, in <lambda>
  File "C:\src\veusz-msvc\veusz\build\veusz_windows_pyinst\out00-PYZ.pyz\veusz.windows.treeeditwindow", line 1163, in _selectWidgetSiblings
AttributeError: 'NoneType' object has no attribute 'children'