Invariant Sometimes Clears All Plots

[krzywon]

Describe the bug
The Invariant Perspective can clear all plots from the application when fitting.

To Reproduce
Steps to reproduce the behavior:

1. Load a data set and run any analysis on it other than Invariant (I loaded AOT core and ran P(r)).
2. Leave all plots open
3. Send the data to the Invariant Perspective
4. Fit both Q regions.
5. Uncheck either Q region and fit again.
6. All plots, invariant or otherwise, are closed and the only plot left is a new plot with the new fit.

Expected behavior
The invariant plot should be refreshed, but other plots should be unaffected.

SasView version (please complete the following information):

• Version: 5.0.6 (maybe earlier, but untested), 6.0.0b2

Additional context
Add any other context about the problem here.

[rozyczko]

InvariantPerspective.py lines 361 and then 390

            if self.low_extrapolation_plot and not low_calculation_pass:
                   (...)
                    reactor.callFromThread(self._manager.filesWidget.closePlotsForItem, item)
                    reactor.callFromThread(self._model_item.removeRow, item.row())

this would cause it?