PanelModel: Fix crash after window resize, fixes #16933 (#16942)

public/app/features/dashboard/state/PanelModel.ts

@@ -324,7 +324,7 @@ export class PanelModel {
   }
 
   hasTitle() {
-    return !!this.title.length;
+    return this.title && this.title.length > 0;
   }
 
   destroy() {