Fixed crash in `ScrollContainer` _notification

(cherry picked from commit ee81979f901e096fab84109b3e66aa1238d1abd2)

scene/gui/scroll_container.cpp

@@ -257,7 +257,9 @@ void ScrollContainer::_notification(int p_what) {
 	};
 
 	if (p_what == NOTIFICATION_READY) {
-		get_viewport()->connect("gui_focus_changed", this, "_gui_focus_changed");
+		Viewport *viewport = get_viewport();
+		ERR_FAIL_COND(!viewport);
+		viewport->connect("gui_focus_changed", this, "_gui_focus_changed");
 	}
 
 	if (p_what == NOTIFICATION_SORT_CHILDREN) {