[YevheniiKotyrlo]

Summary:

ReactScrollViewHelper.findNextFocusableView reads host.getChildAt(0).id without checking for the child, so a focus search that reaches a scroll view whose content view has already been removed throws from ReactScrollView.focusSearch:

java.lang.NullPointerException: Attempt to invoke virtual method 'int android.view.View.getId()' on a null object reference
  at com.facebook.react.views.scroll.ReactScrollViewHelper.findNextFocusableView
  at com.facebook.react.views.scroll.ReactScrollView.focusSearch

#53046 reports it while a screen with a focused TextInput in a ScrollView navigates back; it was closed as stale, and main still dereferences the child. The search now returns null when there is no content view, so focusSearch keeps the platform's answer.

Changelog:

[ANDROID] [FIXED] - Crash in a ScrollView's keyboard focus search after its content view has been removed

Test Plan:

• New Robolectric test ReactScrollViewHelperFocusSearchTest: a vertical and a horizontal scroll view with no content view, the Fabric lookup returning a next focusable. On main both fail with the NullPointerException above, thrown at the getChildAt(0).id read; with this change both return null.
• ./gradlew :packages:react-native:ReactAndroid:testDebugUnitTest --tests "com.facebook.react.views.scroll.*" -PenableWarningsAsErrors=true passes, and ktfmt is clean on both files.