Every model with selectable number of shells larger than 1 throws an error, e.g. using core-multishell with more than 1 shell.
Trying to use PD one the shell no2, 3, etc gives the error:

ERROR: Traceback (most recent call last): File "sas\qtgui\Perspectives\Fitting\PolydispersityWidget.py", line 248, in File "sas\qtgui\Perspectives\Fitting\PolydispersityWidget.py", line 308, in onPolyComboIndexChange File "C:\SasView-6.1.0\sasmodels\sasview_model.py", line 816, in set_dispersion raise ValueError("%r is not a dispersity or orientation parameter" ValueError: 'thickness[n_shells]' is not a dispersity or orientation parameter.

If you want to reproduce the error:

1. select a model like sphere>core_multi_shell
2. choose number of shell to be larger than 1
3. Tick PD and set parameter for shell no 2, you need to set distribution function away from default Gaussian to obtain the error.

Version: Sasview 6.1 WindowsInstaller, Windows 11, Version 6.1 release and 6.1.1dev19+g260ab6d56 (that fixes some PD issues #3505)