wxGUI/iclass: fix closing scatter plot pane via x button

[tmszi]

Describe the bug
Supervised Classification Tool scatter plots pane prints error message when closed.

To Reproduce
Steps to reproduce the behavior:

1. Launch Supervised Classification Tool wxGUI g.gui.iclass
2. On the left Plots pane combobox widget choose Scatter plots item
3. On the Scatter plots toolbar activate Add scatter plot tool
4. On the Select imagery group dialog from the name of imagery group listbox widget items choose lsat7_2000
5. Hit Create/edit group... button widget
6. On the Create or edit image group dialog, type into Select existing subgroup or enter name of new subgroup combobox widget new subgroup name e.g. test and select some maps from the list to be part of the new cretaed subgroup test (lsat7_2000_10, lsat7_2000_20, lsat7_2000_30, lsat7_2000_40) and hit OK button
7. Hit OK button on the Create or edit image group dialog
8. On the next Add scatter plots dialog choose from the x axis combobox widget items lsat7_2000_10 raster map, and from the y axis combobox widget items choose lsat7_2000_20 raster map and hit Add button
9. Repeat point 9. but with another maps, from the x axis combobox widget items choose lsat7_2000_30 raster map, and from the y axis combobox widget items choose lsat7_2000_40 raster map and hit Add button
10. On the Add scatter plots dialog hit OK button
11. Try to close scatter plot pane via x button
12. See error

Traceback (most recent call last):
  File "/usr/lib/python3.10/site-packages/wx/lib/agw/aui/framemanager.py", line 8448, in OnRender
    art.DrawCaption(dc, self._frame, part.pane.caption, part.rect, part.pane)
  File "/usr/lib/python3.10/site-packages/wx/lib/agw/aui/dockart.py", line 593, in DrawCaption
    btns = pane.CountButtons()
  File "/usr/lib/python3.10/site-packages/wx/lib/agw/aui/framemanager.py", line 1715, in CountButtons
    if isinstance(wx.GetTopLevelParent(self.window), AuiFloatingFrame):
RuntimeError: wrapped C/C++ object of type ScatterPlotWidget has been deleted

Expected behavior
Supervised Classification Tool scatter plots pane should not prints error message when closed.

System description:

• Operating System: all
• GRASS GIS version: all

GRASS nc_spm_08_grass7/landsat:~ > python3 -c "import sys, wx; print(sys.version); print(wx.version())"
3.10.13 (main, Sep 16 2023, 22:24:59) [GCC 12.3.1 20230526]
4.2.0 gtk3 (phoenix) wxWidgets 3.2.2.1