Today Xcode refused to show previews for one view, displaying "Cannot preview this file" on top of the previews canvas.
This time it's showing the error "SomeUnitTest.swift must belong to at least one target in the current scheme in order to use previews". I have the feeling that this error is incorrect and masks a problem elsewhere, because unittests most certainly should not belong to any other target than the test target.
I have been able to temporarily fix the problem as follows: