Muon analysis crash when using default grouping with no runs

[AnthonyLim23]

Description of work.

To test:

1. Go to the grouping tab
2. Press default
3. It used to crash, you now get a pop up

Test 2:
Open muon analysis
Set instrument to EMU
Load some data
Change the instrument to MUSR
Go to grouping tab
Click default - crash

Fixes #29705.

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Reviewer

Please comment on the following (full description):

Code Review

• Is the code of an acceptable quality?
• Does the code conform to the coding standards?
• Are the unit tests small and test the class in isolation?
• If there is GUI work does it follow the GUI standards?
• If there are changes in the release notes then do they describe the changes appropriately?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed?
• Do the changes handle unexpected situations, e.g. bad input?
• Has the relevant (user and developer) documentation been added/updated?

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[Matt-Cumber]

Code looks good and it fixes the bug. Just one very minor comment

[martyngigg]

Is it worth us trying to get this into the build tonight for testing?

[AnthonyLim23]

I have already made a package and sent it to them

[martyngigg]

Cool. Thanks.