modedit: clear the pcb when closing the window

Before closing the window in the footprint editor, we clear the existing
pcb, undo/redo stack and associated memory to prevent callbacks
double-freeing on destruction.

Fixes: lp:1784504
* https://bugs.launchpad.net/kicad/+bug/1784504

pcbnew/footprint_edit_frame.cpp

@@ -503,7 +503,12 @@ void FOOTPRINT_EDIT_FRAME::OnCloseWindow( wxCloseEvent& Event )
     }
 
     if( IsGalCanvasActive() )
+    {
+        GetGalCanvas()->SetEventDispatcher( NULL );
         GetGalCanvas()->StopDrawing();
+    }
+
+    Clear_Pcb( false );
 
     //close the editor
     Destroy();