Fix error trace introduced to Specviz model fitting

[rosteen]

Description

Fixes an error trace when selected a subset in the model fitting plugin in Specviz, introduced in #951 due to a last minute change putting code back in for Cubeviz.

[codecov]

Codecov Report

Merging #970 (fb1d327) into main (987f797) will increase coverage by 0.10%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #970      +/-   ##
==========================================
+ Coverage   69.30%   69.41%   +0.10%     
==========================================
  Files          69       69              
  Lines        4946     4947       +1     
==========================================
+ Hits         3428     3434       +6     
+ Misses       1518     1513       -5     

Impacted Files	Coverage Δ
...igs/default/plugins/model_fitting/model_fitting.py	29.29% <0.00%> (-0.10%)	⬇️
...configs/default/plugins/data_tools/file_chooser.py	69.14% <0.00%> (+3.42%)	⬆️

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[pllim]

Note to self: This affects unreleased version, so no need for change log.

Seems uncontroversial enough. Let's get this in!