Incorrect parameters used when switching from fit to prefit without changes in parameters

[ZhaoTianPu]

To replicate the issue: run fit, but do not hit fit -> prefit button, directly go to prefit and hit plot spectrum.

The expected behavior is that the sweep with the parameters in prefit panel is generated and plotted, but the plot remains the one obtained with the fitted parameters.

Possibly need to trigger the update parameter after fit run.