Fix VSI crash when selecting Scale or Cut Filter when "builtin" node is selected

[AntonPiccardoSelg]

Fixes #11947

Issue

The pipeline browser in the VSI has an initial node called builtin (not sure why it is selectable at all). If this node is selected and subsequently Cut or Scale is pressed, Mantid used to crash.

The cause of the crash is related to us querrying for an Active Source when there is none (basically, because the builtin node is neither a source or a filter). The issue arises here, when we pass the Active Source (which is nullptr) to thect ObjectBuilders createFilter method

Solution

In the filter callbacks we need to hedge for empty Active Sources and not do anything if we encounter such a situation.

For Tester:

Please find an Ubunutu installer here: //olympic/babylon5/Scratch/Anton/VSI/11947

Release Notes

• Release note changes can be found here

Implemented Tests

• Note that this is a VSI GUI bugfix and currently not being unit tested

Functional Testing

1. Load a sample workspace into the VSI
2. Select the builin node in the pipeline browser (the icon at the beginning of the pipeline).
   • You should see that the properties browser changes its display
3. Now that it is selected. Press Scale
   • Confirm that nothing happens, especially that Mantid does not crash
4. Press Cut
   • Confirm that nothing happens, especially that Mantid does not crash

[mantid-builder]

Thanks for the changes. Here's a few things that we'll use to start the review process:

Main Reviewer

Please comment on the following:

Pull Request

• Does the title summarize the changes, i.e. it's not the automatically generated one?
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• Are there adequate testing instructions?
• A link to the updated release notes has been provided or no release note updates were necessary?

Code Review

• Is the code of an acceptable quality?
  • Does the code conform to the coding standards? Is it well structured with small focussed classes/methods/functions?
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Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
  • How do the changes handle unexpected situations, e.g. bad input?
• Has the relevant documentation been added/updated?
  • Is user-facing documentation written in a user-friendly manner?
  • Has developer documentation been updated if required?

---

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Second Reviewer (@gatekeepers)

• Have adequate tests, both success & failure cases, been performed?,
• Has the code been reviewed?,
• Ready to go? Hit Merge!

[matthew-d-jones]

Test carried out as described in the PR description with a simple workspace created with CreateMDWorkspace.
No documentation update required as this is a bug fix.