Prevent crash when a python script tries to use pcbnew too early.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/15414

pcbnew/python/scripting/pcbnew_scripting_helpers.cpp

@@ -2,7 +2,7 @@
  * This program source code file is part of KiCad, a free EDA CAD application.
  *
  * Copyright (C) 2012 NBEE Embedded Systems, Miguel Angel Ajo <[email protected]>
- * Copyright (C) 1992-2022 KiCad Developers, see AUTHORS.txt for contributors.
+ * Copyright (C) 1992-2023 KiCad Developers, see AUTHORS.txt for contributors.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
@@ -77,6 +77,13 @@ void ScriptingSetPcbEditFrame( PCB_EDIT_FRAME* aPcbEditFrame )
 }
 
 
+void ScriptingOnDestructPcbEditFrame( PCB_EDIT_FRAME* aPcbEditFrame )
+{
+    if( s_PcbEditFrame == aPcbEditFrame )
+        s_PcbEditFrame = nullptr;
+}
+
+
 BOARD* LoadBoard( wxString& aFileName )
 {
     if( aFileName.EndsWith( KiCadPcbFileExtension ) )