Replacing workspace while it's open in Slice Viewer causes crash

[jclarkeSTFC]

Found during smoke testing for the 6.9 release, but also present in 6.8.

To Reproduce

• Run this script:

from mantid.simpleapi import *

md_4D = CreateMDWorkspace(Dimensions=4, Extents=[0,2,-1,1,-1.5,1.5,-0.25,0.25], Names="H,K,L,E", Frames='HKL,HKL,HKL,General Frame',Units='r.l.u.,r.l.u.,r.l.u.,meV')
FakeMDEventData(InputWorkspace=md_4D, UniformParams='5e5') # 4D data
FakeMDEventData(InputWorkspace=md_4D, UniformParams='1e5,0.5,1.5,-0.5,0.5,-0.75,0.75,-0.1,0.1') # 4D data

# Add a non-orthogonal UB
expt_info = CreateSampleWorkspace()
md_4D.addExperimentInfo(expt_info)
SetUB(Workspace='md_4D', c=2, gamma=120)

# make a 3D MDEvent workspace by integrating over all E
md_3D = SliceMD(InputWorkspace='md_4D', AlignedDim0='H,0,2,100', AlignedDim1='K,-1,1,100', AlignedDim2='L,-1.5,1.5,100')

# Create a peaks workspace and fake data in 3D MD
CreatePeaksWorkspace(InstrumentWorkspace='md_3D', NumberOfPeaks=0, OutputWorkspace='peaks')
CopySample(InputWorkspace='md_3D', OutputWorkspace='peaks', CopyName=False, CopyMaterial=False, CopyEnvironment=False, CopyShape=False)
AddPeakHKL(Workspace='peaks', HKL='1,0,1')
AddPeakHKL(Workspace='peaks', HKL='1,0,0')

• Open Slice Viewer with the md_4D workspace
• Run the script again without closing the Slice Viewer window. Sometimes the window will close at this point, which is probably what's meant to happen, but I can get it to stay open intermittently by dragging it around and changing focus back to the Workbench window. If the window closes then try again until it stays open after running the script.
• Try and overlay peaks
• Sad Mantid

Platform/Version (please complete the following information):

• OS: Windows 10 and macOS, probably all OSs
• Mantid Version 6.8 and 6.9 (smoke testing)

[jhaigh0]

If you, run the script, open the slice viewer, overlay peaks, the run the script again, it resets to the default view (doesn't close) and is in a state where there's an exception every time you mouse over the plot.