Description of work

This PR fixes a bug on the Muon Analysis interface when switching between normal fitting mode and TF Asymmetry fitting. The solution was to check if the tf asymmetry fit function can be parsed before attempting to get individual functions from its structure.

Fixes #37392

To test:

1. Open Muon Analysis interface
2. Select MUSR instrument
3. Load 62260
4. Go to Fitting tab
5. Right click and add function GausOsc in the function browser
6. Tick Simultaneous fit over
7. Change fitting mode from "Normal" to "TF Asymmetry"
8. There should be no error!

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