Fix null pointer error

Summary: Turns out this can be null in some rare circumstances and can cause a crash in the fragment tracker

Reviewed By: aigoncharov

Differential Revision: D50562123

fbshipit-source-id: 4dfaecfde69359a85b778fc5ff0a4a3a8165a875

android/src/main/java/com/facebook/flipper/plugins/uidebugger/descriptors/ViewPagerDescriptor.kt

@@ -23,6 +23,9 @@ object ViewPagerDescriptor : ChainedDescriptor<ViewPager>() {
 
   override fun onGetActiveChild(node: ViewPager): Any? {
     val child = node.getChildAt(node.currentItem)
+    if (child == null) {
+      return null
+    }
     val fragment = FragmentTracker.getFragment(child)
     if (fragment != null) {
       return fragment