Help I have the following error in an activity that lists data in a ListView

The error that is thrown is the following:

  

E / AndroidRuntime: FATAL EXCEPTION: main       Process: com.example.bastiancorps.geoassistance, PID: 17445       java.lang.RuntimeException: Unable to start activity ComponentInfo {com.example.bastiancorps.geoassistance / com.example.bastiancorps.geoassistance.productoActivity}:   java.lang.NullPointerException: Attempt to invoke virtual method   'com.google.firebase.database.DatabaseReference   com.google.firebase.database.DatabaseReference.child (java.lang.String) '   on a null object reference

This is the code of the activity:

public class productoActivity extends AppCompatActivity {
    private List<producto> listaproductos = new ArrayList<producto>();
    ArrayAdapter<producto> arrayAdapterproducto;
    private EditText codigop, nombrep, preciop, stockp, tiendap;
    private FirebaseDatabase midb;
    private DatabaseReference midr;
    private ProgressDialog progressDialog;
    private FirebaseAuth mAuth;
    private ListView milistap;
    producto productosected;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_producto);
        listarDatosproducto();
        codigop = (EditText) findViewById(R.id.ETcodigop);
        nombrep = (EditText) findViewById(R.id.ETnombrep);
        preciop = (EditText) findViewById(R.id.ETpreciop);
        stockp = (EditText) findViewById(R.id.ETstockp);
        tiendap = (EditText) findViewById(R.id.ETtiendap);
        milistap = (ListView) findViewById(R.id.listadeproductos);
        progressDialog = new ProgressDialog(this);
        mAuth = FirebaseAuth.getInstance();
        midb = FirebaseDatabase.getInstance();
        midr = FirebaseDatabase.getInstance().getReference();

        milistap.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                productosected = (producto) parent.getItemAtPosition(position);
                //codigop.setText(productosected.getCodigo());
                nombrep.setText(productosected.getNombre());
                preciop.setText(productosected.getPrecio());
                stockp.setText(productosected.getStock());
                tiendap.setText(productosected.getTienda());
            }
        });
    }

    private void listarDatosproducto() {
        midr.child("producto").addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                listaproductos.clear();
                for (DataSnapshot datasnap : dataSnapshot.getChildren()){
                    producto p = datasnap.getValue(producto.class);
                    listaproductos.add(p);
                    arrayAdapterproducto = new ArrayAdapter<producto>(productoActivity.this, android.R.layout.simple_list_item_1, listaproductos);
                    milistap.setAdapter(arrayAdapterproducto);
                }

            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }

this is the Xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="12dp"
    tools:context=".productoActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_horizontal"
        android:layout_margin="10dp"
        android:text="Ingrese un producto"
        android:textSize="22dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        tools:ignore="MissingConstraints"
        tools:layout_editor_absoluteY="0dp" />

    <EditText
        android:id="@+id/ETcodigop"
        android:layout_width="336dp"
        android:layout_height="35dp"
        android:layout_marginEnd="8dp"
        android:background="@drawable/txt_color"
        android:hint="codigo"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.6"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.109"
        tools:ignore="MissingConstraints" />

    <EditText
        android:id="@+id/ETnombrep"
        android:layout_width="336dp"
        android:layout_height="35dp"
        android:background="@drawable/txt_color"
        android:hint="nombre del producto"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.207"
        tools:ignore="MissingConstraints" />

    <EditText
        android:id="@+id/ETpreciop"
        android:layout_width="337dp"
        android:layout_height="35dp"
        android:layout_marginEnd="8dp"
        android:background="@drawable/txt_color"
        android:hint="precio de venta"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.615"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.296"
        tools:ignore="MissingConstraints" />

    <EditText
        android:id="@+id/ETstockp"
        android:layout_width="336dp"
        android:layout_height="35dp"
        android:background="@drawable/txt_color"
        android:hint="Stock"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.38"
        tools:ignore="MissingConstraints" />

    <EditText
        android:id="@+id/ETtiendap"
        android:layout_width="336dp"
        android:layout_height="35dp"
        android:layout_marginEnd="8dp"
        android:background="@drawable/txt_color"
        android:hint="tienda"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.55"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.474"
        tools:ignore="MissingConstraints" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="center_horizontal"
        android:layout_margin="10dp"
        android:text="Lista de productos"
        android:textSize="22dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.586"
        tools:ignore="MissingConstraints" />

   <ListView
       android:id="@+id/listadeproductos"
       android:layout_width="match_parent"
       android:layout_height="match_parent"></ListView>

</LinearLayout>