To reproduce the crash:
1. Have at least two tab groups, with only one tab in a tab group.
2. Right click on the tab label (for the single tab) -> move to new
   window.
-> the multi-notebook destroys the notebook, with a crash in
   gtk_notebook_detach_tab() on the line after the call to
   gtk_container_remove(), since it tries to access
   notebook->priv->remove_in_detach on the destroyed notebook.