InvertMask algroithm loses workspace Instrument. (on wish?)

[abuts]

This issue was originally TRAC 6116

Tried on wish instrument though suspected generic problem

1. Load wish file (e.g. Wish00016748 from system tests)
2. Show instrument go to mask and select an mask
3. Apply mask (unnecessary but shows masked area of the workspace)
4. Try to run Save As Mask to workspace/Save as ROI to workspace --> works fine and a workspace with name MaskWorkspace appears

(btw, Save as ROI to workspace should produce workspace named ROIWorkspace)
5) SaveCalFile on MaskWorkspace works fine
6) InvertMask from MaskWorkspace to ROIWorkspace works fine
7) SaveCalFile on ROIWorkspace ->

Error:
Error in execution of algorithm SaveCalFile:
There is no instrument associated with workspace 'ROIWorkspace'

[abuts]

@NickDraper (2013-01-28T09:23:19):
Moved at the code freeze for release 2.4

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@NickDraper (2013-04-29T09:49:52):
Moved to r2.6 at the end of r2.5

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@NickDraper (2013-07-26T13:55:00):
Moved to backlog at the code freeze for R2.6

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@NickDraper (2014-02-14T11:04:58):
Bulk move to assigned at the introduction of the triage step

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