Filter and store data in kotlin

Hey I have data coming from server. I have Topic data class inside that I have Categories class, basically I want to store some value. I did this successfully and I want to know is there any better way of doing that in terms of performance, storage, time and space complexity in kotlin idiomatic way. Can someone guide me on this.

I added SubTopic data class dummy is very small amount, but in actually in big size of list is coming from server.

main.kt

fun main() {
    val response = createData()
    response.items?.let { categories ->
        val filteredList = categories.filter { it.categories != null }
            .flatMap { flatCategories ->
                flatCategories.categories!!.forEach { eachCategories ->
                    eachCategories.subTopic?.forEach {
                        it.priceId = flatCategories.id
                    }
                }
                flatCategories.categories
            }

        filteredList.forEach {
            println(it)
        }
    }
}


fun createData() = ResponseClass(
    listOf(
        Topic(
            "xyz",
            "vivek",
            listOf(
                Categories("21", "physical", listOf(SubTopic("1", "abc"), SubTopic("2", "bjhef"))),
                Categories("2211", "mind", listOf(SubTopic("1", "abc"), SubTopic("2", "bjhef"))),
                Categories("22131", "motorized", listOf(SubTopic("1", "abc"), SubTopic("2", "bjhef"))),
                Categories("2134124", "coordination", listOf(SubTopic("1", "abc"), SubTopic("2", "bjhef"))),
                Categories("211243", "animal-supported", listOf(SubTopic("1", "abc"), SubTopic("2", "bjhef"))),
            )
        )
    )
)

ResponseClass.kt

data class ResponseClass(
    val items: List<Topic>? = null
)

Topic.kt

data class Topic(
    val id: String? = null,
    val name: String? = null,
    val categories: List<Categories>? = null
)

Categories.kt

data class Categories(
    val id: String? = null, val title: String? = null, val subTopic: List<SubTopic>? = null
)

SubTopic.kt

data class SubTopic(
    val id: String? = null, val title: String? = null, var priceId: String? = null
)