Fix Step Scan GUI issues found during unscripted testing

[mantid-builder]

Original Reporter: Russell Taylor

• The pixel mask combobox isn't restricted to the correct workspace type (e.g. it shows table workspaces, which will lead to an uncaught exception if selected).
• The GUI becomes unresponsive if things fail in the runStepScanAlg() method in an unexpected way, as the Qt::WA_TransparentForMouseEvents flag does not get unset.
• The above issue is triggered if the input workspace is deleted manually.
• Similar problems also occur if you delete the table and then try to change the plot.
• With live data, MantidPlot locks up if you happen to hit 'Start' while not in a run (until the run does start).
• If masking the data, hitting 'Start' twice lead to an error:

AttributeError: 'NoneType' object has no attribute 'setAxisTitle'
     at line 11 in '`<Interface>`'

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Keywords: LiveData

[mantid-builder]

This issue was originally trac ticket 8290