fix[devtools]: check if fiber is unmounted before trying to highlight (#26983)

For React Native environment, we sometimes spam the console with
warnings `"Could not find Fiber with id ..."`.

This is an attempt to fix this or at least reduce the amount of such
potential warnings being thrown.

Now checking if fiber is already unnmounted before trying to get native
nodes for fiber. This might happen if you try to inspect an element in
DevTools, but at the time when event has been received, the element was
already unmounted.