the date class from the real time database is not filled in - firebase

Question

I can't fill in the date class from the database. I don't understand why it doesn't work, everything seems to be written correctly.
In the initUser function, the date of the User Model class is filled in, but if you write the USER variable to println, it will be empty. Below is the code of the MainActivity class, which has the initUser function
class MainActivity : AppCompatActivity() {
private lateinit var btn_settings: ImageButton
private lateinit var btn_add_friend: ImageButton
private lateinit var btn_search_main: ImageView
private lateinit var search_main: EditText
private lateinit var nickname_main: TextView
private lateinit var Id_main: TextView
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
initFirebase()
initUser()
init()
initFunc()
btn_settings.setOnClickListener {
val intent = Intent(this, SettingsActivity::class.java)
startActivity(intent)
}
btn_search_main.setOnClickListener {
if (search_main.text.toString() == "Happy") {
val intent = Intent(this, psh::class.java)
startActivity(intent)
}
}
nickname_main.text = USER.username
Id_main.text = CURRENT_UID
println(USER)
}
private fun initFunc() {
if (AUTH.currentUser != null) {
} else {
val intent = Intent(this, LoginActivity::class.java)
startActivity(intent)
}
}
private fun init(){
btn_settings = findViewById(R.id.btn_settings)
btn_add_friend = findViewById(R.id.btn_add_friend)
btn_search_main = findViewById(R.id.btn_search_main)
search_main = findViewById(R.id.search_main)
nickname_main = findViewById(R.id.nickname_main)
Id_main = findViewById(R.id.Id_main)
}
private fun initUser(){
REF_DATABASE_ROOT.child(NODE_USERS).child(CURRENT_UID)
.addListenerForSingleValueEvent(AppValueEventListener{
USER = it.getValue(UserModel()::class.java) ?:UserModel()
})
}
fun initFirebase(){
AUTH = FirebaseAuth.getInstance()
REF_DATABASE_ROOT = FirebaseDatabase.getInstance("https://ert-d167-default-rtdb.europe-west1.firebasedatabase.app").reference
USER = UserModel()
CURRENT_UID = AUTH.currentUser?.uid.toString()
}
}
class AppValueEventListener (val onSuccess:(DataSnapshot)->Unit) : ValueEventListener{
override fun onCancelled(error: DatabaseError) {
}
override fun onDataChange(snapshot: DataSnapshot) {
onSuccess(snapshot)
}
}
data class UserModel(
var id: String = "",
var username: String = ""
)
If you insert the USER variable into print, it will be User Model(id=, username=)
The data in the database is