Stope event propagation when closing project.

Fixes https://gitlab.com/kicad/code/kicad/issues/8638

kicad/kicad_manager_frame.cpp

@@ -360,6 +360,9 @@ bool KICAD_MANAGER_FRAME::canCloseWindow( wxCloseEvent& aEvent )
     // If any of them cancel then we need to cancel closing the KICAD_MANAGER_FRAME.
     if( CloseProject( true ) )
     {
+        // Don't propagate event to frames which have already been closed
+        aEvent.StopPropagation();
+
         return true;
     }
     else