## Summary
- Adds a null check before calling
`fabricSuspendOnActiveViewTransition()` in the Fabric renderer's
`suspendOnActiveViewTransition` export
- Prevents crashes on hosts where `nativeFabricUIManager` does not yet
implement `suspendOnActiveViewTransition`
## Test plan
- Verified the change compiles correctly
- Hosts with `suspendOnActiveViewTransition` implemented continue to
work as before
- Hosts without `suspendOnActiveViewTransition` no longer crash when
view transitions are active
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