How to map from DTO to domain when have nullable field in API and don't need it on domain model

I get a complex model from server with Retrofit. I simplified it to make it more obvious.
Two fields are obligatory, but bottomSheet is optional. The problem is that I have to make all the fields of domain model BottomSheetData nullable even though I don't need them nullable.

DTO models:

@Serializable
data class PrivilegeWidgetDto(
    val widgetHeader: String,
    val widgetText: String,
    val bottomSheet: BottomSheetDto? = null,
)

@Serializable
data class BottomSheetDto(
    val bottomSheetText: String? = null,
    val bottomSheetHeader: String? = null,
)

Domain models:

@Serializable
data class PrivilegeWidget(
    val widgetHeader: String? = null,
    val widgetText: String? = null,
    val bottomSheet: BottomSheetData? = null,
)

@Serializable
data class BottomSheetData(
    //want to make these fields not nullable
    val bottomSheetText: String? = null, 
    val bottomSheetHeader: String? = null,
)

I use extension function to map:

fun PrivilegeWidgetDto.mapToDomain() : PrivilegeWidget {
    return PrivilegeWidget(
        widgetHeader = this.widgetHeader,
        widgetText = this.widgetText,
        bottomSheet = BottomSheetData(
            bottomSheetHeader = this.bottomSheet?.bottomSheetHeader,
            bottomSheetText = this.bottomSheet?.bottomSheetText
        )
    )
}

The problem is that though bottomSheet in PrivilegeWidget may be null, its own fields should not be nullable (if bottomSheet != null it always contains bottomSheetHeader and bottomSheetText). Otherwise I have to work with nullable values even though I don't need them.

Is these any good practice to handle it? What if I make sth like this - are these solution ok?

    bottomSheet = BottomSheetData_(
        bottomSheetHeader = this.bottomSheet?.bottomSheetHeader!!,
        bottomSheetText = this.bottomSheet?.bottomSheetText!!
    )



    bottomSheet = BottomSheetData_(
        bottomSheetHeader = this.bottomSheet?.bottomSheetHeader ?: "",
        bottomSheetText = this.bottomSheet?.bottomSheetText ?: ""
    )

UPD:

If I define bottomSheetHeader and bottomSheetText as non-nullable in both DTO and Domain objects I get compiler error in mapper and face the same problem:

> Only safe (?.) or non-null asserted (!!.) calls are allowed on a
> nullable receiver of type BottomSheetDto?

If bottomSheet is nullable mapper treats its fields as nullable too even if they are non-nullable. So I have to solve same problems with nullability in mapper.

One way to achieve this is by using a secondary constructor in the BottomSheetData class to enforce non-nullability of its fields:

data class BottomSheetData(
    val bottomSheetText: String,
    val bottomSheetHeader: String
) {
    constructor(bottomSheetText: String?, bottomSheetHeader: String?) : this(
        bottomSheetText ?: "",
        bottomSheetHeader ?: ""
    )

Now, when you create an instance of BottomSheetData, you can use the secondary constructor to handle the case where bottomSheet is nullable

fun PrivilegeWidgetDto.mapToDomain(): PrivilegeWidget {
    return PrivilegeWidget(
        widgetHeader = this.widgetHeader,
        widgetText = this.widgetText,
        bottomSheet = this.bottomSheet?.let {
            BottomSheetData(
                bottomSheetText = it.bottomSheetText,
                bottomSheetHeader = it.bottomSheetHeader
            )
        }
    )
}

add 2 separate extension functions like this and change bottomSheetHeader and bottomSheetText as non-nullable

fun PrivilegeWidgetDto.mapToDomain() : PrivilegeWidget {
    return PrivilegeWidget(
        widgetHeader = this.widgetHeader,
        widgetText = this.widgetText,
        bottomSheet = bottomSheet?.mapToDomain()
    )
}

fun BottomSheetDto.mapToDomain(): BottomSheetData {
   return BottomSheetData(
        bottomSheetHeader = bottomSheetHeader,
        bottomSheetText = bottomSheetText
    )
}

Compiler complains about Only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type BottomSheetDto? because in your exention function this.bottomSheet is already nullable and any other operation will require null checks.