fix(devtools): guard useLayoutEffect against null ref in ContextMenu

The useLayoutEffect in ContextMenu accesses ref.current without
checking for null. When portalContainer is missing or items is empty,
the component returns null (no portal rendered), leaving ref.current
as null and causing a crash on the subsequent .contains() call.

Guard the effect with the same early-return condition used by the
render path (portalContainer == null || items.length === 0) so the
effect is a no-op when no portal is mounted.

Author: fresh3nough

packages/react-devtools-shared/src/devtools/ContextMenu/ContextMenu.js

@@ -74,7 +74,13 @@ export default function ContextMenu({
   );
 
   useLayoutEffect(() => {
-    const menu = ((ref.current: any): HTMLElement);
+    const menu = ref.current;
+
+    // Match the early-return condition below. If neither of these
+    // is true, menu being null would be a bug.
+    if (portalContainer == null || items.length === 0) {
+      return;
+    }
 
     function hideUnlessContains(event: Event) {
       if (!menu.contains(((event.target: any): Node))) {