Tree Tabs: crash when closing a tab

[toofar]

Version info: Hacked up integration branch which includes the tree-tabs-integration branch commit https://github.com/qutebrowser/qutebrowser/commit/9bf2bae60f6daf631729cb901c845e6c48da4778.

Does the bug happen if you start with --temp-basedir?: Don't know how to reproduce.

Description Got this stack trace (and browser crash) while closing a tab in a hurry. Don't quite know what I was doing, I think I might have just opened the tab, maybe from an undo.

Traceback (most recent call last):
  File "/home/user/projects/stable-qutebrowser-6/qutebrowser/mainwindow/tabbedbrowser.py", line 616, in _on_window_close_requested
    self.close_tab(widget)
  File "/home/user/projects/stable-qutebrowser-6/qutebrowser/mainwindow/tabbedbrowser.py", line 474, in close_tab
    self._remove_tab(tab, add_undo=add_undo, new_undo=new_undo)
  File "/home/user/projects/stable-qutebrowser-6/qutebrowser/mainwindow/treetabbedbrowser.py", line 87, in _remove_tab
    self._add_undo_entry(tab, idx, new_undo)
  File "/home/user/projects/stable-qutebrowser-6/qutebrowser/mainwindow/treetabbedbrowser.py", line 141, in _add_undo_entry
    entry = _TreeUndoEntry.from_node(node, 0)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/projects/stable-qutebrowser-6/qutebrowser/mainwindow/treetabbedbrowser.py", line 47, in from_node
    parent_uid = node.parent.uid
                 ^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'uid'

How to reproduce

Not 100% sure, I definitely have closed many tabs without issue before. Maybe a timing issue?