Make plt.close() call thread safe to pass interactive backend tests

Prevent multiple attempts to close the frame by setting to FigureManagerWx.frame to None when closed by GUI.
When closed by plt.close() call, the frame is closed in a threadsafe way using wx.CallAfter( frame.Close )

Author: tohc1

lib/matplotlib/backends/backend_wx.py

@@ -971,7 +971,14 @@ def _onClose(self, event):
         _log.debug("%s - onClose()", type(self))
         self.canvas.close_event()
         self.canvas.stop_event_loop()
-        Gcf.destroy(self.figmgr)
+        # set FigureManagerWx.frame to None to prevent repeated attempts to close
+        # this frame from with the FigureManagerWx.destroy()
+        if self.figmgr is not None:
+            self.figmgr.frame = None 
+            # remove figure manager from Gcf.figs
+            Gcf.destroy(self.figmgr)
+        #carry on with wx close event propagation, frame & children destruction
+        event.Skip()
 
     def GetToolBar(self):
         """Override wxFrame::GetToolBar as we don't have managed toolbar"""
@@ -988,8 +995,9 @@ def Destroy(self, *args, **kwargs):
         # MPLBACKEND=wxagg python -c 'from pylab import *; plot()'.
         if self and not self.IsBeingDeleted():
             super().Destroy(*args, **kwargs)
-            if self.toolbar is not None:
-                self.toolbar.Destroy()
+            # This should not be necessary if the close event is allowed to propagate.
+            #if self.toolbar is not None:
+            #    self.toolbar.Destroy()
         return True
 
 
@@ -1038,7 +1046,9 @@ def destroy(self, *args):
         _log.debug("%s - destroy()", type(self))
         frame = self.frame
         if frame:  # Else, may have been already deleted, e.g. when closing.
-            frame.Destroy()
+            # as this can be called from non gui thread from plt.close use 
+            # wx.CallAfter to esnure thread safety.
+            wx.CallAfter( frame.Close )
 
     def full_screen_toggle(self):
         # docstring inherited