is there a nice way to get from a Set<String?> to a Set<String> then?

you could filter and then perform an unchecked cast if performance is of utmost priority

what's the type safe solution?

set.filterNotNull().toSet()

is concise enough, but decidedly not as performant if your data size is real large

can you use that in a sequence as well?

you can also do

.filterNotNullTo(mutableSetOf())

to prevent the set->list->set thing