fix event.relatedTarget fallback logic for firefox (#19607)
fix event.relatedTarget fallback logic for firefox (#19607)

* fix event.relatedTarget fallback logic for firefox

* check if relatedTarget is undefined for fallback
ArunTesco/react,terminatorheart/react,trueadm/react,billfeller/react,billfeller/react,acdlite/react,acdlite/react,facebook/react,billfeller/react,tomocchino/react,facebook/react,camsong/react,yungsters/react,cpojer/react,cpojer/react,trueadm/react,camsong/react,billfeller/react,mosoft521/react,acdlite/react,yungsters/react,ArunTesco/react,facebook/react,mosoft521/react,mosoft521/react,camsong/react,trueadm/react,billfeller/react,cpojer/react,terminatorheart/react,terminatorheart/react,mosoft521/react,acdlite/react,billfeller/react,ArunTesco/react,facebook/react,trueadm/react,camsong/react,camsong/react,trueadm/react,terminatorheart/react,cpojer/react,yungsters/react,yungsters/react,camsong/react,cpojer/react,mosoft521/react,facebook/react,yungsters/react,terminatorheart/react,cpojer/react,tomocchino/react,cpojer/react,yungsters/react,mosoft521/react,yungsters/react,tomocchino/react,tomocchino/react,acdlite/react,acdlite/react,acdlite/react,billfeller/react,mosoft521/react,trueadm/react,facebook/react,facebook/react,trueadm/react,tomocchino/react,tomocchino/react,camsong/react,terminatorheart/react,tomocchino/react