Fix doc theme not changing when its hidding

(cherry picked from commit 9a5cce35e609adb42dce4a5217ac41cbb609441f)

editor/editor_help.cpp

@@ -1456,7 +1456,7 @@ void EditorHelp::_notification(int p_what) {
 			_update_doc();
 		} break;
 		case NOTIFICATION_THEME_CHANGED: {
-			if (is_visible_in_tree()) {
+			if (is_inside_tree()) {
 				_class_desc_resized();
 			}
 		} break;