Contacts don't get deleted reliably

Steps to reproduce:

• Open private chat with contact
• Click to delete them
• Click + to add new contact

Expected:

• Contact gets deleted

Actually:

(Briar:3): Handy-CRITICAL **: 17:49:00.203: hdy_stackable_box_set_visible_child: assertion 'contains_child' failed
Traceback (most recent call last):
  File "/app/share/briar-gtk/briar_gtk/widgets/undo_notification.py", line 57, in _on_destroy
    action()
  File "/app/share/briar-gtk/briar_gtk/containers/main_window.py", line 99, in _timeout_action
    self.show_sidebar()
  File "/app/share/briar-gtk/briar_gtk/containers/main_window.py", line 85, in show_sidebar
    self.chat_view.hide()
AttributeError: 'NoneType' object has no attribute 'hide'

Maybe we shouldn't do this "Undo" thing at all...