Describe the bug

Found during manual testing for the 6.9 release (#36702). Trying to open a plot on a workspace that is being updated with data from the fake ISIS_Histogram instrument causes an unhandled exception. In Mantid version 6.8 this would log an error message but not trigger the unhandled exception, so it appears to be a regression.

To Reproduce

Go through the following steps from the live data manual testing instructions to get the live data running:

1. https://developer.mantidproject.org/Testing/LiveData/LiveDataTests.html#set-up-the-test-facility
2. https://developer.mantidproject.org/Testing/LiveData/LiveDataTests.html#isis-histogram
3. https://developer.mantidproject.org/Testing/LiveData/LiveDataTests.html#starting-a-live-data-session (just steps 1 and 2 here)
4. https://developer.mantidproject.org/Testing/LiveData/LiveDataTests.html#live-histogram-data

Then, with live data running, right click on the workspace in the ADS and try to open a plot (select a single spectrum to plot). This should cause an unhandled exception.

Expected behavior

In previous versions an error was logged but it didn't cause the unhandled exception.

Platform/Version (please complete the following information):

• Mantid Version - introduced since version 6.8.