message: Fix crash when clearing scene.  Weren't canceling frame in progress.
diff: @@ -1403,6 +1403,9 @@ void MainWindow::buildMainMenuEdit()
  ImGui::SameLine(ImGui::GetWindowWidth()-(8*ImGui::GetFontSize()));
  if (ImGui::Button("Yes, clear it")) {
  + // Cancel any in-progress frame
  + frame->cancelFrame();
  + frame->waitOnFrame();
  frame->remove("world");
  lightsManager->clear();
  if(animationWidget) {
  