[DevTools] prevent StyleX plugin from throwing when inspecting CSS (#26364)

## Summary

An error might happen when we try to read the CSS rules, but the
stylesheet does not allow so (often happens on production).

Before:
<img width="713" alt="image"
src="https://user-images.githubusercontent.com/1001890/224376546-024f7a32-d314-4dd1-9333-7e47d96a2b7c.png">

After:

<img width="504" alt="image"
src="https://user-images.githubusercontent.com/1001890/224376426-964a33c4-0677-4a51-91c2-74074e4dde63.png">


## How did you test this change?

Built a fb version and tested locally (see above screenshot)

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Co-authored-by: Hendrik Liebau <mail@hendrik-liebau.de>