Your spinner shows no value because the data is not retrieved yet from the DB.

So what you should do is create a callback from FirebaseHelper to IncomeFragment

First, create a callback

interface FirebaseHelperCallback {
    fun dataRetrievedFromDB(data: List<String>)
}

Second, create a variable, setter function and call dataRetrievedFromDB function in FirebaseHelper class

class FirebaseHelper(val db: DatabaseReference) {
   private lateinit var firebaseHelperCallback: FirebaseHelperCallback

    fun setFirebaseHelperCallback(firebaseHelperCallback: FirebaseHelperCallback) {
        this.firebaseHelperCallback = firebaseHelperCallback
    }

    ...

    private fun fetchData(snapshot: DataSnapshot, bankData: ArrayList<String>) {
       bankData.clear()
       val name: BankData? = snapshot.getValue(BankData::class.java)
       bankData.add(bankData.toString())
       firebaseHelperCallback.dataRetrievedFromDB(bankData)
    }
}

Lastly, implement the callback in IncomeFragment

class IncomeFragment: Fragment(), FirebaseHelperCallback {


   override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
   ): View? {
      ...
      helper.setFirebaseHelperCallback(this)
   }

   ...

   override fun dataRetrievedFromDB(data: List<String>) {
        spinner.setAdapter(
           activity?.let {
             ArrayAdapter<String>(
                it,
                android.R.layout.simple_list_item_1,
                data
             )
           }
        )
   }
}

I have added firebase database and the data also gets saved into the firebase realtime database, but it doesnot gets shown into the spinner, my spinner is showing no value.If theres some other way?

This is the code of my fragment:

class IncomeFragment: Fragment(){

//FAB
lateinit var addBankFab: FloatingActionButton
lateinit var addBankText: TextView

//Spinner
lateinit var spinner: Spinner
lateinit var dropDown: TextView

//Firebase Database
lateinit var mAuth: FirebaseAuth
lateinit var mIncomeDatabase: DatabaseReference
lateinit var mBankDatabase: DatabaseReference
lateinit var helper: FirebaseHelper

//Recycler View
lateinit var recyclerView: RecyclerView

//TextView
lateinit var incomeTotal : TextView

//Update Edit Text
lateinit var editAmount: EditText
lateinit var editType: EditText
lateinit var editNote: EditText
lateinit var editBankName:EditText

//Button for update and delete
lateinit var btnUpdate: Button
lateinit var btnDelete: Button
lateinit var btnSave : Button
lateinit var btnCancel:Button

//Data item Value

lateinit var type: String
lateinit var note: String
var amount: Int = 0
lateinit var post_key:String
lateinit var name:String
lateinit var ac:String

override fun onCreateView(
    inflater: LayoutInflater, container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    // Inflate the layout for this fragment
    val myview= inflater.inflate(R.layout.fragment_income, container, false)
    mAuth= FirebaseAuth.getInstance()
    val mUser = mAuth.currentUser
    val uid = mUser?.uid

    addBankFab=myview.findViewById(R.id.add_bank_fab)
    addBankText=myview.findViewById(R.id.add_bank_text)


    spinner=myview.findViewById(R.id.incomeSpinner)

    mIncomeDatabase =
        FirebaseDatabase.getInstance().reference.child("IncomeData").child(uid.toString())
    incomeTotal=myview.findViewById(R.id.income_txt_result)

    mBankDatabase= FirebaseDatabase.getInstance().reference.child("BankData").child(uid.toString())


    helper = FirebaseHelper(mBankDatabase)

    spinner.setAdapter(
        activity?.let {
            ArrayAdapter<String>(
                it,
                android.R.layout.simple_list_item_1,
                helper.retrieve()
            )
        }
    )

    recyclerView=myview.findViewById(R.id.incomeRecyclerView)
    val layoutManager = LinearLayoutManager(activity)
    layoutManager.reverseLayout = true
    layoutManager.stackFromEnd = true
    recyclerView.setHasFixedSize(true)
    recyclerView.layoutManager = layoutManager

    mIncomeDatabase.addValueEventListener(object : ValueEventListener {
        override fun onDataChange(dataSnapshot: DataSnapshot) {
            var totalvalue :Int = 0
            for (ds in dataSnapshot.children) {
                val data: Data = ds.getValue<Data>(Data::class.java)!!
                totalvalue+=data.amount
                val stTotalValue:String = valueOf(totalvalue)

                incomeTotal.text = stTotalValue+".00"

            }

        }

        override fun onCancelled(databaseError: DatabaseError) {}
    })


    return myview

}




private fun insertBank() {
    val bankdialog = AlertDialog.Builder(activity)
    val inflater = LayoutInflater.from(activity)
    val view: View = inflater.inflate(R.layout.add_bank_layout, null)
    bankdialog.setView(view)
    val dialog = bankdialog.create()
    dialog.setCancelable(false)

    val BankName = view.findViewById<EditText>(R.id.account_bank)
    val BankAc = view.findViewById<EditText>(R.id.account_number)
    val btnSaveBank = view.findViewById<Button>(R.id.btn_save_bank)
    val btnCancelBank = view.findViewById<Button>(R.id.btn_cancel_bank)

    btnSaveBank.setOnClickListener {

        val name = BankName.text.toString().trim()
        val ac = BankAc.text.toString().trim()


        if (TextUtils.isEmpty(name)) {
            BankName.error = "Required Field..."
            [email protected]
        }

        if (TextUtils.isEmpty(ac)) {
            BankAc.error = "Required Field..."
            [email protected]
        }

        val bankacno = ac.toInt()


        val id: String? = mBankDatabase.push().key
        val mDate: String = DateFormat.getDateInstance().format(Date())
        val bankdata = EmiData(bankacno, name, id, mDate)
        mBankDatabase.child(id.toString()).setValue(bankdata)
        Toast.makeText(activity, "DATA ADDED", Toast.LENGTH_SHORT).show()


        dialog.dismiss()

    }

    btnCancelBank.setOnClickListener {
        dialog.dismiss()

    }

    dialog.show()
}

override
fun onStart() {
    super.onStart()
    val options = FirebaseRecyclerOptions.Builder<Data>()
        .setQuery(mIncomeDatabase, Data::class.java)
        .setLifecycleOwner(this)
        .build()
    val firebaseRecyclerAdapter: FirebaseRecyclerAdapter<Data, MyViewHolder> =
        object : FirebaseRecyclerAdapter<Data, MyViewHolder>(options) {
            override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): MyViewHolder {
                return MyViewHolder(
                    LayoutInflater.from(parent.context)
                        .inflate(R.layout.income_recycler_data, parent, false)
                )
            }

            override fun onBindViewHolder(holder: MyViewHolder, position: Int, model: Data) {

                holder.setType(model.type)
                holder.setAmount(model.amount)
                holder.setDate(model.date)
                holder.setNote(model.note)

                holder.mView.setOnClickListener {

                    post_key= getRef(position).key.toString()

                    type=model.type
                    note=model.note
                    amount=model.amount

                    updateDataItem()
                    }

            }

        }
    recyclerView.adapter = firebaseRecyclerAdapter
}

class MyViewHolder(itemView: View) : ViewHolder(itemView) {
    val mView = itemView

    fun setType(type: String) {
        val mType = mView.findViewById<TextView>(R.id.type_txt_income)
        mType.text = type
    }
    fun setNote(note: String) {
        val mNote = mView.findViewById<TextView>(R.id.note_txt_income)
        mNote.text = note
    }
    fun setDate(date: String) {
        val mDate = mView.findViewById<TextView>(R.id.date_txt_income)
        mDate.text = date
    }
    fun setAmount(amount: Int) {
        val mAmount = mView.findViewById<TextView>(R.id.amount_txt_income)
        val stAmount = valueOf(amount)
        mAmount.text = stAmount
    }



}

private fun updateDataItem() {
    val mydialog = AlertDialog.Builder(activity)
    val inflater = LayoutInflater.from(activity)
    val myView: View = inflater.inflate(R.layout.update_data_item, null)
    mydialog.setView(myView)
    val dialog = mydialog.create()

    val editAmount = myView.findViewById<EditText>(R.id.amount_edit)
    val editType = myView.findViewById<EditText>(R.id.type_edit)
    val editNote = myView.findViewById<EditText>(R.id.note_edit)

    //Set data to edit text..

    editType.setText(type)
    editType.setSelection(type.length)

    editNote.setText(note)
    editNote.setSelection(note.length)

    editAmount.setText(valueOf(amount))
    editAmount.setSelection(valueOf(amount).length)

    val btnUpdate = myView.findViewById<Button>(R.id.btn_update)
    val btnDelete = myView.findViewById<Button>(R.id.btn_delete)

    btnUpdate.setOnClickListener {
        type = editType.text.toString().trim()
        note = editNote.text.toString().trim()

        var mdAmount = amount.toString()
        mdAmount = editAmount.text.toString().trim { it <= ' ' }
        val myAmount = mdAmount.toInt()

        val mDate: String = DateFormat.getDateInstance().format(Date())
        val data = Data(myAmount, type, note, post_key, mDate)

        mIncomeDatabase.child(post_key).setValue(data)

        dialog.dismiss()


    }

    btnDelete.setOnClickListener {
        mIncomeDatabase.child(post_key).removeValue()

        dialog.dismiss()
    }
    dialog.show()
}
}

This is my firebase helper:

class FirebaseHelper(val db: DatabaseReference) {
private var saved: Boolean? = null

//SAVE
fun save(bankData: BankData?): Boolean? {
    saved = if (bankData == null) {
        false
    } else {
        try {
            db.child("name").push().setValue(bankData)
            true
        } catch (e: DatabaseException) {
            e.printStackTrace()
            false
        }
    }
    return saved
}

//READ
fun retrieve(): ArrayList<String> {
    val bankNames = ArrayList<String>()
    db.addChildEventListener(object : ChildEventListener {
        override fun onChildAdded(
            dataSnapshot: DataSnapshot,
            s: String?
        ) {
            fetchData(dataSnapshot, bankNames)
        }

        override fun onChildChanged(
            dataSnapshot: DataSnapshot,
            s: String?
        ) {
            fetchData(dataSnapshot, bankNames)
        }

        override fun onChildRemoved(dataSnapshot: DataSnapshot) {}
        override fun onChildMoved(
            dataSnapshot: DataSnapshot,
            s: String?
        ) {
        }

        override fun onCancelled(databaseError: DatabaseError) {}
    })
    return bankNames
}

private fun fetchData(
    snapshot: DataSnapshot,
    bankData: ArrayList<String>
) {
    bankData.clear()
    val name: BankData? = snapshot.getValue(BankData::class.java)
    bankData.add(bankData.toString())
    }


}