Using lateinit in Kotlin

Question

fun main() {
    val lateData = LateData()
    with(lateData) {
        lateinitNumber = 10
        nullableNumber.printLog()
        lateinitNumber.printLog()
    }
}
class LateData {
    var nullableNumber: Int? = null
    lateinit var lateinitNumber: Int
}

I was intended to use lateinit keyword, but lateinit in LateData class shows red underline. how can I fix this?