Kotlin: How to get user Email/Username from firebase on Android - algorithm

Question

I am creating a project whereby a user database is automatically created on signup for the first time. It functions well, however, i want the information on the database to display the User's Email or Name and not uid as shown below.
the only reason i am using uid is because i have failed to successfully manage to call a function that enables me to display it.
here is my code:
they belong in an Object activity so the first 2 code examples are object codes
initiate the firebase setup
object FirestoreUtil {
private val FirestoreInstance: FirebaseFirestore by lazy { FirebaseFirestore.getInstance() }
private val currentUserDocRef: DocumentReference
get() = FirestoreInstance.document(
"users/${FirebaseAuth.getInstance().currentUser?.uid
?: throw NullPointerException("UID is null.")}"
)
2.THIS is used on the signup button to create a new users database on launching;
fun InitCurrentUserIfFirsttime(onComplete: () -> Unit) {
currentUserDocRef.get().addOnSuccessListener { documentSnapshot ->
if (!documentSnapshot.exists()) {
val newUser = User(
FirebaseAuth.getInstance().currentUser?.displayName ?: "",
"", "",null, mutableListOf()
)
currentUserDocRef.set(newUser).addOnSuccessListener {
onComplete()
}
}else{
onComplete()
}
}
}
3.My data class;
data class User(val name: String,
val bio: String,
val age: String,
val profilePicturePath: String?,
val registrationTokens: MutableList<String>) {
constructor(): this("", "", "",null, mutableListOf())
}
4.from my signup button;
auth.createUserWithEmailAndPassword(signuptv.text.toString(), signupPassword.text.toString())
.addOnCompleteListener(this) { task ->
if (task.isSuccessful) {
FirestoreUtil.InitCurrentUserIfFirsttime {
startActivity(Intent(this, LogInActivity::class.java))
finish()
}
} else {
Toast.makeText(baseContext, "Authentication failed.",
Toast.LENGTH_SHORT).show()
}
}