Fix NPE in ui-debugger when child isn't found

Summary:
Fixes exception that happens occasionally on iOS  {F1484099525}

triggered here: https://www.internalfb.com/code/fbsource/[033ecc866dfb]/xplat/sonar/desktop/plugins/public/ui-debugger/components/visualizer/Visualization2D.tsx?lines=558

Working theory: activeChildIndex is out of bounds. Added error attribution to make sure it ends up in logview

Reviewed By: antonk52

Differential Revision: D56011805

fbshipit-source-id: f511ff7dd22588d301427913d250fdd6cdd558ad

desktop/plugins/public/ui-debugger/components/visualizer/Visualization2D.tsx

@@ -562,7 +562,14 @@ function hitTest(node: NestedNode, mouseCoordinate: Coordinate): NestedNode[] {
     let children = node.children;
 
     if (node.activeChildIdx != null) {
-      children = [node.children[node.activeChildIdx]];
+      const activeChild = node.children[node.activeChildIdx];
+      if (activeChild == null) {
+        console.error(
+          `[ui-debugger] activeChildIdx not found for ${node.name}: ${node.activeChildIdx} not within ${node.children.length}`,
+        );
+      } else {
+        children = [activeChild];
+      }
     }
     const offsetMouseCoord = offsetCoordinate(mouseCoordinate, nodeBounds);
     let anyChildHitRecursive = false;