Fix crash if Shared Element ID is invalid

[mrousavy]

Fixes a hard-crash on iOS if the specified Shared Element ID could not be found/invalid nativeID has been set.

Instead, the Shared Element Transition, which could not locate the specified View by it's nativeID, will just be ignored.

[mrousavy]

I just noticed that when one nativeID (either source or target) is invalid/cannot be found, the Android app gets stuck. So the similar issue can be reproduced on Android.

I'm guessing this has something to do with OptimisticViewFinder:: find, since that cannot find the view via the sync approach (ReactFindViewUtil.findView(root.view, nativeId)), and therefore falls back to adding a view found listener - which will never be called, because the view doesn't exist.

So either we only try the sync approach, and if it can't be found, we skip this shared element transition (I'm guessing this would require every shared element transition to have a waitForRender: true, which makes sense anyways doesn't it?), or we somehow add some complex timeout logic.

With the timeout logic I still don't think this would make sense, as the navigation and therefore the whole app is completely stalled during this time.

[yogevbd]

@mrousavy When defining shared element transition we already wait for render. I think we should also throw an informative exception in such case in addition to the check you've added.

[mrousavy]

@yogevbd I see. We could use the LogBox to show a warning