feat: fix missing initial var in useRef and how to doc

docs/how_tos/migrate-frontend-app.md

@@ -129,7 +129,7 @@ With the exception of any custom scripts, replace the `scripts` section of your
 
 ```json
   "scripts": {
-    "dev": "PORT=YOUR_PORT openedx dev",
+    "dev": "PORT=YOUR_PORT PUBLIC_PATH=/YOUR_APP_NAME openedx dev",
     "i18n_extract": "openedx formatjs extract",
     "lint": "openedx lint .",
     "lint:fix": "openedx lint --fix .",
@@ -139,6 +139,7 @@ With the exception of any custom scripts, replace the `scripts` section of your
 ```
 
 - Replace `YOUR_PORT` with the desired port, of course.
+- Replace  `YOUR_APP_NAME` with the basename used on your site.config, not doing this will result in only the root route working.
 - Note that `fedx-scripts` no longer exists, and has been replaced with `openedx`.
 
 > [!TIP]

runtime/slots/widget/iframe/hooks.ts

@@ -127,7 +127,7 @@ export function dispatchUnmountedEvent() {
  */
 export function useElementSize() {
   // Holds a reference to the ResizeObserver
-  const observerRef = useRef<ResizeObserver>();
+  const observerRef = useRef<ResizeObserver | null>(null);
 
   const [dimensions, setDimensions] = useState({ width: 0, height: 0 });
   const [offset, setOffset] = useState({ x: 0, y: 0 });