subset plugin: fix IndexError

[kecnry]

Description

This pull request fixes the IndexError raised in the subset plugin when modifying a subset (color in plot options, for example) while "Create New" is selected.

Fixes #1693

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?
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

Base: 87.16% // Head: 87.16% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (52ed625) compared to base (d1506cf).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1700   +/-   ##
=======================================
  Coverage   87.16%   87.16%           
=======================================
  Files          95       95           
  Lines        9908     9910    +2     
=======================================
+ Hits         8636     8638    +2     
  Misses       1272     1272           

Impacted Files	Coverage Δ
...igs/default/plugins/subset_plugin/subset_plugin.py	97.91% <100.00%> (+0.02%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.

[duytnguyendtn]

Tested against main and can confirm it fixes on my end. Thanks for the fix, and even better that you encapsulated it in a test!

[pllim]

LGTM. Thanks!