Add nullcheck in MimeHandlerViewEmbedder::DidFinishNavigation

The RFH accessed in a DCHECK might be null if the navigation never reached the ReadyToCommitNavigation state.

Bug: 1133115
Change-Id: I57472efb15d4acb375c5a6980ca5b661bb08f53a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2569637
Auto-Submit: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833274}

extensions/browser/guest_view/mime_handler_view/mime_handler_view_embedder.cc

@@ -106,8 +106,10 @@ void MimeHandlerViewEmbedder::ReadyToCommitNavigation(
 
 void MimeHandlerViewEmbedder::DidFinishNavigation(
     content::NavigationHandle* handle) {
-  if (frame_tree_node_id_ != handle->GetFrameTreeNodeId())
+  if (!render_frame_host_ ||
+      frame_tree_node_id_ != handle->GetFrameTreeNodeId()) {
     return;
+  }
   // We should've deleted the MimeHandlerViewEmbedder at this point if the frame
   // is sandboxed.
   DCHECK(!render_frame_host_->IsSandboxed(