Fix a crash when enable a plugin uses `make_mesh_previews`

The bug happens when plugin tree is propagating mouse events
(so it is blocked), but EditorProgress's dtor will make main editor
focused and call update_plugins immediately
 which will update the blocked tree.

editor/editor_plugin_settings.cpp

@@ -245,7 +245,7 @@ EditorPluginSettings::EditorPluginSettings() {
 	plugin_list->set_column_custom_minimum_width(3, 80 * EDSCALE);
 	plugin_list->set_column_custom_minimum_width(4, 40 * EDSCALE);
 	plugin_list->set_hide_root(true);
-	plugin_list->connect("item_edited", callable_mp(this, &EditorPluginSettings::_plugin_activity_changed));
+	plugin_list->connect("item_edited", callable_mp(this, &EditorPluginSettings::_plugin_activity_changed), CONNECT_DEFERRED);
 
 	VBoxContainer *mc = memnew(VBoxContainer);
 	mc->add_child(plugin_list);