i got this error and cant resolve it: Expected a List while deserializing, but got a class java.lang.String; - firebase

Question

i kown that is sunday but i got this problem, and i dont have any idea about how i fix it.
ps: the other questions in stackoverflow are in java, im using kotlin, and are about map error, not list.
my class User
#Parcelize
class User (val primeiroLogin:Boolean, val tutorialComplete:Boolean, val
username:String, val uid:String, val viewed:ArrayList<String>) : Parcelable {
constructor() : this(true,false,"","", ArrayList())
}
most part of the activity where i got the error
var viewed = ArrayList<String>()
private fun selectCategories(){
val ref = FirebaseDatabase.getInstance().getReference("/users")
ref.addListenerForSingleValueEvent(object: ValueEventListener{
override fun onCancelled(p0: DatabaseError) {
}
override fun onDataChange(p0: DataSnapshot) {
var cUid = ""
val user = FirebaseAuth.getInstance().currentUser
user?.let {
val uid = user.uid
cUid = uid
}
p0.children.forEach{
val user = it.getValue(User::class.java)
if(user?.uid == cUid){
viewed = user.viewed
}
}
}
})
}
private fun fetchCategories(){
val ref = FirebaseDatabase.getInstance().getReference("/categories")
ref.addListenerForSingleValueEvent(object: ValueEventListener{
override fun onCancelled(p0: DatabaseError) {
}
override fun onDataChange(p0: DataSnapshot) {
val adapter = GroupAdapter<ViewHolder>()
p0.children.forEach{
val category = it.getValue(Category::class.java)
if(category != null){
for (i in 0..viewed.size) {
if(category.real_name != viewed[i]){
adapter.add(categories(category))
}
}
}
}
adapter.setOnItemClickListener { item, view ->
val categories = item as categories
val intent = Intent(view.context, LearningFirstLibras::class.java)
intent.putExtra(CATEGORY_KEY, categories.category)
startActivity(intent)
}
new_words_categories.adapter = adapter
}
})
.....
thank you guys!