QENS Fitting crash when fixing IsoDiffRot parameter

[robertapplin]

Reported by: Mohamed Aounae

Description
There is a crash on the QENS Fitting interface when attempting to Fix all parameters from the EditLocalParameter dialog. It should be possible to fix this parameter without a crash.

To Replicate

1. Open QENS Fitting interface
2. Go to Convolution tab
3. Load a Sample and Resolution file
4. Select Fit Type IsoDiffRot
5. Click in the radius parameter box and select ...
6. Click the Set dropdown button and choose Remove all ties
7. Click the Set dropdown button and choose Fix all
8. Click Ok
9. Crash!