Check for NULL frame tree root.

content/browser/web_contents/web_contents_impl.cc

Index: content/browser/web_contents/web_contents_impl.cc
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index 81c00b1d78d4ff6ec6eabbc3a8f2235c00ed1554..11a66b1fbb570707ca499724f1af97fef7840784 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -2483,6 +2483,11 @@ void WebContentsImpl::OnUpdateFaviconURL(
 }
 
 FrameTreeNode* WebContentsImpl::FindFrameTreeNodeByID(int64 frame_id) {
+  // TODO(nasko): Remove this check once we move to creating the root node
+  // through RenderFrameHost creation.
+  if (!frame_tree_root_.get())
+    return NULL;
+
   FrameTreeNode* node = NULL;
   std::queue<FrameTreeNode*> queue;
   queue.push(frame_tree_root_.get());