Closing the widget, then opening a new one leads to issues with annotation

[jluethi]

When closing a classifier widget, opening a new image and then loading the first classifier for that second image, the annotation is not working as expected. It seems to overwrite some annotations with something else (e.g. set all annotations to class 1, even if another class was selected).

I wonder whether there's an issue with the first widget still being active in the background and we'd have to deal with closing it better.

In the meantime, the workaround is to close napari whenever the widget is closed. If the second image is opened in a fresh napari session, everything works as expected.

[jluethi]

Closed by #46