[BUG] Vue error inconsistent for user-supplied label for new component in model fitting

[MatthewPortman]

Jdaviz component

Other (explain in Description below)

Description

If a user supplies a label to a component in model fitting, the label does not display a vue error ("label is already in use") even though the label is being used.

This may be fixed in a future PR (see JDAT-5463) intended to address incrementing user-supplied labels but this issue should be double checked when that PR is implemented.

How to Reproduce

1. Using Specviz/Cubeviz open Model Fitting plugin in GUI
2. Modify the label for this new model component.
3. Click ADD COMPONENT.
4. At this point, the label has not changed but no vue error pops up stating that the label is already in use.
5. Click ADD COMPONENT again to see a traceback.