Spectrum Viewer Export Tabs disable when no stacks are available

[MikeSullivan7]

Issue

Close #2242

Description

When a stack is modified from the main window (e.g. deleted), the spectrum viewer now checks to see if there is an imagestack available to display. If no data is available (i.e. all data has been deleted), then the Export Tab area is disabled. This prevents a KeyError from occurring when changing ExportTab while there is no imagestack.

Testing

make check

Acceptance Criteria

1. Open MI and load in data
2. Open the Spectrum Viewer
3. In the Main Window, delete the data you just loaded in
4. Check in the Spectrum Viewer that no sample stacks are available and that the Export Tabs have been disabled, and therefore no error occurs.
5. In the Main Window, load in some more data.
6. Check that the Export Tabs have now been re-enabled and function as normal.

Documentation

Will add release note

[coveralls]

coverage: 73.152% (+0.008%) from 73.144%
when pulling 00c0b69 on 2242_spectrum_viewer_keyerror_no_stacks
into 27975ce on main.

[samtygier-stfc]

This is a good solution. I've tested that it resolves the issue.