breakManager: Fix a potential null object dereference

breakManager: Fix a potential null object dereference

There’s no `else` branch above, so it’s possible for
`this._notification` to be `null` at the bottom of the function
(especially if the user has explicitly closed the previous
notification), so handle that.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8280
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3687>