Fix vue warning and potential js error in ScalarProbe.vue

[1isten]

the ScalarProbe.vue causes console warning:

and causes this js error when switching Layout, for example from Quad View to Axial Only:

this PR fix above issues

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[PaulHax]

This is awesome, thank you so much for fixing my bugs. To get this mergred would you mind

• Moving the undefined check for the missing sample.rep to the getBaseSlice() func? The base slice "sample" is basicly invalid if the ref is not defined yet, so better to catch that earler in the reactive chain.
• Change the commit messages to follow semantic commit style. So change them to fix(ScalarProbe): eliminate vue console warning and fix(ScalarProbe): avoid accessing undefined rep
• Be sure to format the code style so Pretter is happy: npm run prettify

[1isten]

Thanks @PaulHax , I created a new PR: #723, can you check?