wxGUI/vdigit: fix close frame if Vector Digitizer is run as standalone app g.gui.vdigit

[tmszi]

trafficstars

Describe the bug The standalone Vector Digitizer app window doesn't close after you hit "power off" toolbar.

To Reproduce Steps to reproduce the behavior:

1. Launch wxGUI Vector Digitizer as standalone app e.g. g.gui.vdigit map=geology
2. Quit Vector Digitizer by click on the "power off" toolbar
3. The Vector Digitizer window doesn't close

Expected behavior Standalone Vector Digitizer app window should be closed after you hit "power off" toolbar.

System description (please complete the following information):

• Operating System: all
• GRASS GIS version: 8.3.dev, 8.2