FRET: FD Fit does not work (v17.03.24)

[NicolaasvanderVoort]

Describe the bug
Selecting FRET:FD Fit does not open a Fit window

To Reproduce
Steps to reproduce the behavior:

1. Load a dataset into ChiSurf
2. Select model: 'FRET: FD (Discrete)
3. Press add fit
4. Nothing happens

Expected behavior
A fit window appears, enabling D0 calibrated lifetime fits

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [win10 x64
• Version 17.03.24

Additional context
Using the Record Movie feature, this command was executed:

cs.add_fit(model_name='FRET: FD (Discrete)', dataset_idx=[1])