wxGUI/vdigit: fix close settings dialog (if no vector map has been selected)

[tmszi]

To Reproduce
Steps to reproduce the behavior:

1. Launch wxGUI
2. Display some vector map e.g. 'railroads'
3. Launch Vector digitizer, 2D view -> Vector digitizer, and choose 'railroads' vector map
4. Quit vector digitizer (click on the Quit toolbar tool)
5. Launch Vector digitizer again, but don't choose 'railroads' vector map
6. Launch Vector digitizer settings dialog (gear wheel icon on the vector digitizer toolbar tool)
7. Hit close button on the vector digitizer settings dialog
8. See error

Error message

Traceback (most recent call last):
  File "/usr/lib64/grass79/gui/wxpython/mapwin/buffered.py",
line 605, in OnPaint

self.pdcVector.DrawToDCClipped(gcdc, rgn)
AttributeError
:
'NoneType' object has no attribute 'DrawToDCClipped'
Traceback (most recent call last):
  File "/usr/lib64/grass79/gui/wxpython/mapwin/buffered.py",
line 605, in OnPaint

self.pdcVector.DrawToDCClipped(gcdc, rgn)
AttributeError
:
'NoneType' object has no attribute 'DrawToDCClipped'