Fix error when switching to another `GridMap` with an item with higher index selected

(cherry picked from commit fe16aecbacaef37b8a56de34c9cb5a69f7a26644)

modules/gridmap/grid_map_editor_plugin.cpp

@@ -904,10 +904,12 @@ void GridMapEditor::update_palette() {
 	}
 
 	if (selected != -1 && mesh_library_palette->get_item_count() > 0) {
-		mesh_library_palette->select(selected);
+		// Make sure that this variable is set correctly.
+		selected_palette = MIN(selected, mesh_library_palette->get_item_count() - 1);
+		mesh_library_palette->select(selected_palette);
 	}
 
-	last_mesh_library = mesh_library.operator->();
+	last_mesh_library = *mesh_library;
 }
 
 void GridMapEditor::edit(GridMap *p_gridmap) {