Backport PR #10972: Protect against undefined delegated label (#12371)

Co-authored-by: Frédéric Collonval <fcollonval@gmail.com>

packages/mainmenu-extension/src/index.ts

@@ -852,7 +852,7 @@ namespace Private {
       ? find(s, value => value.tracker.has(widget!))
       : undefined;
 
-    if (!extender) {
+    if (!extender || !extender[label]) {
       return '';
     } else {
       const count: number = extender.tracker.size;