Open /xwiki/bin/view/Help/Applications/Movies/?useLiveData=true and then look at the Advanced Search panel. The live data appears to be sorted by the unsortable property "poster".
There are two issues:
- when no sort property is specified it should use the first sortable property (because the first property might not be sortable as it's the case with the movies live data)
- the user should be able to remove the wrong sort, even if the property is not sortable. The user can specify the wrong (unsortable) property when writing the live data query, but that doesn't mean then shouldn't be able to correct it from the Advanced Sort panel