[Web LA] Fix null elements error

[m-bert]

Summary

This PR includes the following changes:

1. Start exiting animation only if component is defined (to avoid issues related to null).
2. Call fixElementPosition only when parent is defined.

These checks are expected to eliminate cannot read properties of null error.

Origin of this PR

This PR originates from discussion on discord, where it was reported that layout animations crash on web when using react-navigation and expo-router. Crashes happen when display: none; is set. We are not sure what is the exact cause of this issue. However, our main suspect is MutationObserver inside domUtils.ts. Also, we don't know why element is null, but we believe that it can happen because of this line, since this is the only place where we assign something to _component.

Snippet from discord

The snippet provided on discord looks like this:

    <Animated.View
      className="flex flex-row md:hidden px-3 border-b border-b-zinc-800 h-11"
      entering={FadeIn.duration(600)}
      exiting={FadeOut}
    >

As you can tell it uses Tailwind. Unfortunately it doesn't help much.

Error message

Original error message looks like this:

By looking at this error we were able to identify that the crash happens during exiting animation. We still don't know which one is null in this case - parent or element.

Test Plan

We struggle to prepare a reproduction as it is very rare problem. However, I believe that the changes are effective.

[piaskowyk]

We also need to investigate the cause of the issue. If it's a race condition, it would be beneficial to determine the root cause. If it's undefined, there might be a specific point where the breakdown occurs.