add nullptr check to avoid crash in RCTScrollViewComponentView (#42156)

Summary:
Pull Request resolved: #42156

changelog: [internal]

Add a missing nullptr check to prevent crash if called after component was reused

Reviewed By: fkgozali

Differential Revision: D52572807

fbshipit-source-id: 1b5b26996e562abbcb986865299e02df20b58043

Author: sammy-SC

packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm

@@ -471,6 +471,10 @@ - (BOOL)scrollViewShouldScrollToTop:(UIScrollView *)scrollView
 
 - (void)scrollViewDidScrollToTop:(UIScrollView *)scrollView
 {
+  if (!_eventEmitter) {
+    return;
+  }
+
   _isUserTriggeredScrolling = NO;
   static_cast<const ScrollViewEventEmitter &>(*_eventEmitter).onScrollToTop([self _scrollViewMetrics]);
   [self _updateStateWithContentOffset];