kotlin 変数にnullを代入する

fun main() {

    var str1: String? = null
    var str2: String? = "a"
    str2 = null

}

fun main() {

    var str1: String? = null   

    println(str1.length)

}

error: only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type String?
    println(str1.length)
                ^