list.filter { it.field != null }
    .associateBy({ it.id }, { it.field!! })

!!

list

data class Person(val id: String, val field: Field?)

mapNotNull

Pair?

.toMap()

columns
    .mapNotNull { (id, direction) ->
        if (direction == null) null else id to direction
    }
    .toMap()

(id, dir) -> dir?.let { Pair(id, it) }

map.get

associate

associateBy

.associate { it.id to it.direction!! }