It seems like $labels can also be of type null; however, parameter $labels of Phpml\Metric\ConfusionMa...nerateMatrixWithZeros() does only seem to accept array, maybe add an additional type check?
(
Ignorable by Annotation
)
If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation
The expression return array_flip($labels) could return the type null which is incompatible with the type-hinted return array. Consider adding an additional type-check to rule them out.