Checking if username already exists gives error kotlin - firebase

Question

I translated the code below from java, but I am getting an error in some places, why is it giving an error, is there a point I missed?
val mQuery: Query = firestore.collection("users")
.whereEqualTo("nickname", mUserName)
mQuery.addSnapshotListener(object : EventListener<QuerySnapshot> {
fun onEvent(
documentSnapshots: QuerySnapshot,
e: FirebaseFirestoreException?
) {
for (ds in documentSnapshots) {
if (ds != null) {
val userName: String = document.getString("username")
Log.d(
TAG,
"checkingIfusernameExist: FOUND A MATCH: $userName"
)
Toast.makeText(
this#SignUpActivity,
"That username already exists.",
Toast.LENGTH_SHORT
).show()
}
}
}
})
I've been doing the things described here for 2-3 days, but it keeps throwing an error.Event listeners and docs throw errors.

Answer 1

I translated the code below from java
That's not the correct way of "translating" that code from Java to Kotlin, since that answer provides a solution for getting data only once. So to be able to do that in Kolin programming language please use the following lines of code:
val rootRef = FirebaseFirestore.getInstance()
val allUsersRef = rootRef.collection("all_users")
val userNameQuery = allUsersRef.whereEqualTo("username", "userNameToCompare")
userNameQuery.get().addOnCompleteListener { task ->
if (task.isSuccessful) {
for (document in task.result) {
if (document.exists()) {
Log.d("TAG", "username already exists")
val userName = document.getString("username")
//Do what you need to do with the userName
} else {
Log.d("TAG", "username does not exists")
}
}
} else {
Log.d("TAG", "Error getting documents: ", task.exception)
}
}