Kotlin: Cannot convert Map to JSON

The answer provided by Bitwise DEVS may work, but the explanation is incorrect.

It is not necessary to convert the map into an immutable map - it can even be seen in the screenshot of the question: expected (Mutable)Map means that it can be mutable or immutable.

The real problem here is that the called constructor of JSONObject is defined for Map<Any?, Any?> but the given parameter is of type Map<String!, String!>. We know that String is a subtype of Any?, so why is that a problem?

It is a problem because Map is invariant in the key parameter which means that a Map<A, V> is not subtype of Map<B, V> even if A is subtype of B (and also not if it is the other way around).

The call of toMap does not work because it makes the map immutable, but because it is defined as

fun <K, V> Map<out K, V>.toMap(): Map<K, V>

In the receiver type of toMap the key parameter is annotated with the out modifier, meaning that K is covariant here. This means that a Map<Any, V> in this position can safely be considered as a supertype of Map<String, V>. Additionally, Map is always covariant in its value type, meaning that Map<Any, Any> in this position can safely be considered as a supertype of Map<String, String>.

You could as well just cast remoteMessage.data as Map<Any?, Any?> or use toMutableMap, it would be all the same. It has definitely nothing to do with mutability of the map.

Kotlin documentation on variance: https://kotlinlang.org/docs/generics.html#declaration-site-variance

Kotlin documentation of Map: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-map/

Definition of toMap: https://github.com/JetBrains/kotlin/blob/ea836fd46a1fef07d77c96f9d7e8d7807f793453/libraries/stdlib/src/kotlin/collections/Maps.kt#L600