wxGUI/settings: fix hiding 'Tools' tab for single-window mode

[tmszi]

Describe the bug
When you check checkbox widget Hide 'Tools' tab (wxGUI settings dialog) for single-window mode, wxGUI doesn't launch.

To Reproduce
Steps to reproduce the behavior:

1. Launch wxGUI settings dialog and check checkbox widget Hide 'Tools' tab and Use single-windows mode.
2. Hit Save button
3. Restart wxGUI (close and launch wxGUI)
4. See error

Traceback (most recent call last):
  File "/home/tomas/.local/lib/python3.9/site-packages/wx/core.py", line 3285, in <lambda>
    lambda event: event.callable(*event.args, **event.kw) )
  File "/usr/lib64/grass83/gui/wxpython/wxgui.py", line 95, in show_main_gui
    mainframe = GMFrame(parent=None, id=wx.ID_ANY, workspace=self.workspaceFile)
  File "/usr/lib64/grass83/gui/wxpython/main_window/frame.py", line 164, in __init__
    self.BuildPanes()
  File "/usr/lib64/grass83/gui/wxpython/main_window/frame.py", line 681, in BuildPanes
    notebook = self._auimgr.GetNotebooks()[0]
IndexError: list index out of range

Expected behavior
When you check checkbox widget Hide 'Tools' tab (wxGUI settings dialog) for single-window mode, wxGUI should be launch without error.

System description (please complete the following information):

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

[petrasovaa]

Looks fine, although it is probably more a workaround than a real fix. I just added a small suggestion to fix the variable naming for clarity

[petrasovaa]

Suggested change

	notebook = self._auimgr.GetNotebooks()
	if notebook:
	notebook = notebook[0]
	notebooks = self._auimgr.GetNotebooks()
	if notebooks:
	notebook = notebooks[0]