SCH_MARKER::SerializeToString): protect against null pointer

eeschema/sch_marker.cpp

@@ -2,7 +2,7 @@
  * This program source code file is part of KiCad, a free EDA CAD application.
  *
  * Copyright (C) 2009 Jean-Pierre Charras, jaen-pierre.charras@gipsa-lab.inpg.com
- * Copyright (C) 1992-2023 KiCad Developers, see AUTHORS.txt for contributors.
+ * Copyright (C) 1992-2024 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
@@ -114,7 +114,7 @@ wxString SCH_MARKER::SerializeToString() const
         // exclusion must refer to the parent's KIID, and include the text of the original text
         // item for later look-up.
 
-        if( parent->IsType( { SCH_SYMBOL_T, SCH_LABEL_T, SCH_SHEET_T } ) )
+        if( parent && parent->IsType( { SCH_SYMBOL_T, SCH_LABEL_T, SCH_SHEET_T } ) )
         {
             return wxString::Format( wxT( "%s|%d|%d|%s|%s|%s|%s|%s" ),
                                      m_rcItem->GetSettingsKey(),