mslice plot options causes mantid hard crash when using waterfall and log scale

[MialLewis]

Describe the bug When using waterfall and log scale, opening plot options can cause mantid to hard crash

To Reproduce Steps to reproduce the behavior:

1. From mantid nightly open mslice (also in mslice standalone)
2. Load data MAR21335_Ei60meV.nxs used
3. On cut tab, type from 0 to 10 in along |Q| and -5 to 5 in over DeltaE
4. Click plot over (the more lines overplotted, seems to induce crash faster)
5. Click Waterfall on the cut plot toolbar
6. Right click y axis, set to logarithmic
7. Click settings until it opens (usually crashes on first click).

Exits with code 0xC0000005 (access violation).

Expected behavior No crash.

Mantid nightly 6.3.20220601.1641

[MialLewis]

It looks like this hard crash has been inadvertently fixed by #778. It can not currently be reproduced on release-next in the same way.

It is not completely clear why this PR fixes the bug due to the following inconsistencies:

• This hard crash does not throw with an error message, whereas the bug fixed by #778 invariably does.
• It is possible to get this hard crash on the first opening of the plot options, which is before the line_widget.destroyed signal is emitted.

We will keep an eye on this one going forward, and in the next mantid nightly with an updated mslice SHA.

[MialLewis]

I think this can close now, as we have not seen this since?