Fix crash on missing children

Summary:
T98487412 is firing again. This seems to be an expected case that's not handled
correctly.

thinkofthechildren

Reviewed By: fabiomassimo

Differential Revision: D30539615

fbshipit-source-id: d9625bb0a7b3622e5afb6e768bc525c27fdfb02d

desktop/plugins/public/layout/Inspector.tsx

@@ -299,7 +299,8 @@ export default class Inspector extends Component<Props, State> {
       await this.getNodes([id], options);
     }
     this.updateElement(id, {expanded: true});
-    return this.getNodes(this.elements()[id].children, options);
+    const element: Element | undefined = this.elements()[id];
+    return this.getNodes((element && element.children) || [], options);
   }
 
   async getNodes(