Remove group reference when exporting individual footprints

(Cherry-picked from d923b871d7)

pcbnew/footprint_libraries_utils.cpp

@@ -763,6 +763,9 @@ void PCB_EDIT_FRAME::ExportFootprintsToLibrary( bool aStoreInNewLib, const wxStr
                     resetReference( fpCopy );
                     pi->FootprintSave( libPath, fpCopy );
 
+                    // Remove reference to the group before deleting
+                    fpCopy->SetParentGroup( nullptr );
+
                     delete fpCopy;
                 }
             }

pcbnew/plugins/kicad/pcb_plugin.cpp

@@ -2675,8 +2675,9 @@ void PCB_PLUGIN::FootprintSave( const wxString& aLibraryPath, const FOOTPRINT* a
             footprint->Flip( footprint->GetPosition(), false );
     }
 
-    // Detach it from the board
+    // Detach it from the board and its group
     footprint->SetParent( nullptr );
+    footprint->SetParentGroup( nullptr );
 
     wxLogTrace( traceKicadPcbPlugin, wxT( "Creating s-expr footprint file '%s'." ), fullPath );
     footprints.insert( footprintName,