Resolve android crash on display modal (#35380)

Summary:
From exception logging, found crashes due to `Attempt to invoke virtual method 'int android.view.ViewGroup.getChildCount()' on a null object reference`. Tracing through the stack, it appears the constructor for `ViewGroup` conditionally calls `initializeScrollbarsInternal()`, which in turn calls `getChildCount()`.  However `ReactModalHostView` overrides `getChildCount()`, so `getChildCount()` is called before `ReactModalHostView` constructor completes, resulting in null reference when accessing `mHostView` from `getChildCount()`.

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[Android] [Fixed] - Fix crash on initialize modal

Pull Request resolved: #35380

Test Plan: In the rn-tester project, display a modal.

Reviewed By: javache, cipolleschi

Differential Revision: D41392235

Pulled By: ryancat

fbshipit-source-id: ce78e4d458ad41769e78139ea0a8a038384e830d

ReactAndroid/src/main/java/com/facebook/react/views/modal/ReactModalHostView.java

@@ -120,7 +120,9 @@ public void addView(View child, int index) {
 
   @Override
   public int getChildCount() {
-    return mHostView.getChildCount();
+    // This method may be called by the parent constructor
+    // before mHostView is initialized.
+    return mHostView == null ? 0 : mHostView.getChildCount();
   }
 
   @Override