Magnetic parameter minimum limits not editable some times

[krzywon]

Describe the bug
As noted in #3581, the minimum limits for magnetic parameters are not modifiable at times. This is not 100% repeatable, so this might need further investigation.

#3581 (comment)

To Reproduce
Steps to reproduce the behavior:

1. Load a 2D data file and model that can use magnetic fitting (sphere...)
2. Change to a different model
3. Attempt to modify the lower limit of a magnetic parameter.
4. The minimum limit is not recognized.

Expected behavior
All parameter values should be modifiable at any point outside of during the fit.

SasView version (please complete the following information):

• Version: 6.1.1RC