validatedIndices static method
Filters indices to only those within [0, itemCount).
Returns null when indices is null, preserving the "no explicit
selection" semantic for callers that distinguish between "nothing passed"
and "empty set passed".
Implementation
static Set<int>? validatedIndices(Set<int>? indices, int itemCount) {
if (indices == null) return null;
return indices.where((i) => i >= 0 && i < itemCount).toSet();
}