fix subset histogram bug in plot options

[bmorris3]

Description

When subsets are selected in Plot Options in Imviz/Cubeviz, the stretch histogram update raises an error. This PR adds a return to drop out of the stretch histogram update method when the selected layer is a subset, avoiding the error.

This bug was discovered in #2387, splitting it out so it can be merged before 3.6.2.

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

Patch coverage: 66.66% and project coverage change: +90.60% 🎉

Comparison is base (bbcdcc8) 0.00% compared to head (b5f4287) 90.60%.

Additional details and impacted files

@@            Coverage Diff            @@
##           main    #2393       +/-   ##
=========================================
+ Coverage      0   90.60%   +90.60%     
=========================================
  Files         0      159      +159     
  Lines         0    18189    +18189     
=========================================
+ Hits          0    16480    +16480     
- Misses        0     1709     +1709     

Files Changed	Coverage Δ
...nfigs/default/plugins/plot_options/plot_options.py	93.15% <66.66%> (ø)

... and 158 files with indirect coverage changes

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[kecnry]

Would be great to add a regression test into test_plot_options.test_stretch_histogram if you have time, but otherwise looks good to me!

I am a little surprised that the existing if not self.stretch_function_sync.get('in_subscribed_states') check higher up didn't exclude this scenario (since that is what excludes the UI from being shown... 🤔 ), but an extra check below to avoid crashing doesn't hurt anything.

Thanks for the find and fix!

[rosteen]

Confirmed that it works, thanks!

[haticekaratay]

Looks good to me! Thanks

[cshanahan1]

shall we merge this?

[kecnry]

ah, this was never tagged for backport, I'll try to kickstart it manually

[kecnry]

@meeseeksdev backport to v3.6.x

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

git checkout v3.6.x
git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

git cherry-pick -x -m1 914d6d5330a6e93c7778c00d619d5bef8583aead

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

git commit -am 'Backport PR #2393: fix subset histogram bug in plot options'

4. Push to a named branch:

git push YOURFORK v3.6.x:auto-backport-of-pr-2393-on-v3.6.x

5. Create a PR against branch v3.6.x, I would have named this PR:

"Backport PR #2393 on branch v3.6.x (fix subset histogram bug in plot options)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the Still Needs Manual Backport label once the PR gets merged.

If these instructions are inaccurate, feel free to suggest an improvement.

[kecnry]

@meeseeksdev backport to v3.6.x