Save button throws KeyError when a multiple tab layout is saved using Save Button after closing the Preference window using close button

[vssdeo]

Launch terminator in debug mode -d

1. create a layout with 2 tabs.
2. save layout. Ensure you press close button in preferences and NOT (x) on top
3. open preferences->layout again and selected the newly created layout with 2 tabs.
4. press save

you will get the error below.

-whats happening is that on pressing close button in preferences.

        terminator.reconfigure() is called via on_closebutton_clicked(...)
        which in turn calls configure in notebook.py and clears
        self.last_active_term = {}

        on on pressing save point 4 above, that, eventually describe_layout in container.py gets called for Notebook and while setting it using self.get_nth_page, a KeyError is thrown since the terminal object was cleared as explained above.

        last_active_term.append(self.last_active_term[self.get_nth_page(tabnum)])

Traceback (most recent call last):
  File "/home/user/dev/terminator/terminatorlib/prefseditor.py", line 1615, in on_layoutrefreshbutton_clicked
    current_layout = terminator.describe_layout()
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/dev/terminator/terminatorlib/terminator.py", line 629, in describe_layout
    count = window.describe_layout(count, parent, layout, 0, save_cwd)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/dev/terminator/terminatorlib/container.py", line 336, in describe_layout
    count = child.describe_layout(count, name, global_layout, child_order, save_cwd)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/dev/terminator/terminatorlib/container.py", line 314, in describe_layout
    last_active_term.append(self.last_active_term[self.get_nth_page(tabnum)])
                            ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
KeyError: <terminal.Terminal object at 0x7fc933399a40 (terminatorlib+terminal+Terminal at 0x22195d0)>