使用Kotlin检索数据的Firestore错误

我正在尝试使用MVVM模式从Firebase Firestore检索数据。每次构建都会成功，但是应用无法打开。我无法在代码中找到错误。应用程序可以添加数据，但要检索数据是问题。请帮忙。谢谢

视图模型

import android.util.Log
import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import com.google.firebase.firestore.FirebaseFirestore

import com.karuneshpalekar.firestorepagination.models.Note
import java.lang.Exception
class DataViewModel : ViewModel() {

    companion object{
        private const val  TAG =" VIEWMODEL"
    }
    private val db = FirebaseFirestore.getInstance()
    private val noteref = db.collection("record")

    private val _results = MutableLiveData<Exception>()
    val results: LiveData<Exception>
        get() = _results

    private val _notes = MutableLiveData<List<Note>>()
    val notes: LiveData<List<Note>>
        get() = _notes
    fun addData(note: Note) {
        noteref.add(note).addOnCompleteListener {
            if (it.isSuccessful) {
                Log.w("TAG", "success")
                _results.value = null
            } else {
                _results.value = it.exception
            }
        }
    }
    fun fetchData() {

        noteref.addSnapshotListener{ snapshot,e->

            if (e!=null){
                Log.w(TAG,"Listen failed",e)
                return@addSnapshotListener
            }

            if (snapshot != null) {
               val items = mutableListOf<Note>()
                for (docs in snapshot){
                    val notes  = docs.toObject(Note::class.java)
                    notes.let {
                        items.add(it)
                    }
                    _notes.value = items
                }

            }
        }
    }
}

适配器

import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.recyclerview.widget.RecyclerView
import com.karuneshpalekar.firestorepagination.R
import com.karuneshpalekar.firestorepagination.models.Note
import kotlinx.android.synthetic.main.list_item.view.*

class DataAdapter : RecyclerView.Adapter<RecyclerView.ViewHolder>() {

    private var items= mutableListOf<Note>()
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecyclerView.ViewHolder {
        return DataViewHolder(
            LayoutInflater.from(parent.context).inflate(R.layout.list_item, parent, false)
        )
    }

    override fun getItemCount(): Int {

        return items.size
    }

    override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {

        when (holder) {
            is DataViewHolder -> {
                holder.bind(items[position])
            }
        }


    }

    fun setNote(items:List<Note>){
        this.items = items as MutableList<Note>
        notifyDataSetChanged()
    }



    class DataViewHolder constructor(itemView: View) : RecyclerView.ViewHolder(itemView) {

        private val textName = itemView.text_view_name
        fun bind(note: Note) {
            textName.text = note.name
        }

    }
}

分段

import android.os.Bundle
import androidx.fragment.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.lifecycle.Observer
import androidx.lifecycle.ViewModelProviders
import com.karuneshpalekar.firestorepagination.R
import kotlinx.android.synthetic.main.fragment_recyclerview.*


class RecyclerView : Fragment() {


    private lateinit var viewModel: DataViewModel
    private lateinit var dataadapter : DataAdapter
    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        viewModel = ViewModelProviders.of(this).get(DataViewModel::class.java)
        return inflater.inflate(R.layout.fragment_recyclerview, container, false)
    }

   override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)

       recycler_view.apply {
           dataadapter = DataAdapter()
           adapter = dataadapter
       }


       viewModel.fetchData()

       viewModel.notes.observe(viewLifecycleOwner, Observer {
           dataadapter.setNote(it)
       })

        floating_btn_add.setOnClickListener {
            DialogFragment().show(childFragmentManager, "")

        }
    }
}

DialogFragment-添加数据

class DialogFragment :DialogFragment(){

    private lateinit var viewmodel:DataViewModel


    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        viewmodel = ViewModelProviders.of(this).get(DataViewModel::class.java)
        return inflater.inflate(R.layout.fragment_dialog, container, false)
    }
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setStyle(STYLE_NO_TITLE, android.R.style.Theme_DeviceDefault_Light_Dialog_MinWidth)
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        viewmodel.results.observe(viewLifecycleOwner, Observer {
            val message = if (it == null) {
            getString(R.string.name_added)
        } else {
            getString(R.string.name_error)
        }
            Toast.makeText(requireContext(), message, Toast.LENGTH_SHORT).show()
            dismiss()

        })

        button_add.setOnClickListener {
            val names = edit_text_name.text.toString().trim()
            val note= Note("",names)
            viewmodel.addData(note)

        }

    }

}

RecyclerView XML

<androidx.coordinatorlayout.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    tools:context=".ui.RecyclerView">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recycler_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:padding="@dimen/margin"
        app:layoutManager="androidx.recyclerview.widget.LinearLayoutManager"
        tools:listitem="@layout/list_item"
        />

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/floating_btn_add"
        app:fabSize="normal"
        android:layout_gravity="bottom|end"
        android:layout_margin="@dimen/margin"
        />



</androidx.coordinatorlayout.widget.CoordinatorLayout>

以上是我的项目代码