Kotlin user without nulls

KotlinUserWithoutNulls.kt

data class KotlinUserWithoutNulls(val firstName: String,
                                  // this parameter can't be null
                                  val lastName: String,
                                  val addresses: List<Address> = listOf()) {

    data class Address(val city: String)

    companion object {
        fun fetchFirstCity(user: KotlinUserWithNulls) =
            user.addresses.first().city
    }
}