I am trying to retrieve data from firebase in a recyclerview? Whenever I Click the button the app crashes

Question

This is my Final Year Project Which I tend to submitt in couple of weeks. I'm a starter and trying to learn the java //here is the adapter I created //In here I'm calling the layout where data will retrieved

public class UserListAdapter extends RecyclerView.Adapter< UserListAdapter.UserListViewHolder> {

    ArrayList< UserObject > userList;

    public UserListAdapter(ArrayList< UserObject > userList) {
        this.userList = userList;
    }

    @NonNull
    @Override
    public UserListViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType){
        View layoutView= LayoutInflater.from(parent.getContext()).inflate(R.layout.item_user,null,false);
        RecyclerView.LayoutParams lp= new RecyclerView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,ViewGroup.LayoutParams.WRAP_CONTENT);
        layoutView.setLayoutParams(lp);

        UserListViewHolder rcv= new UserListViewHolder(layoutView);
        return rcv;
    }
    @Override
    public void  onBindViewHolder (@NonNull UserListViewHolder holder , int position){
        holder.mName.setText(userList.get(position).getName());
        holder.mPhone.setText(userList.get(position).getPhone());

    }
    @Override
    public int getItemCount(){
        return userList.size();
    }

    public class UserListViewHolder extends RecyclerView.ViewHolder{
        public TextView mName,mPhone;
        public UserListViewHolder(View view){

            super(view);
            mName= view.findViewById(R.id.name);
            mPhone= view.findViewById(R.id.phone);

        }
    }
}

// here is the object I created

public class UserObject {
    String name,phone;
    public UserObject(String name, String phone){
        this.name = name;
        this.phone = phone;
    }

    public String getPhone() { return phone;    }
    public String getName() { return name;    }

    public void setName(String name) {
        this.name = name;
    }

    public void setPhone(String phone) {
        this.phone = phone;
    }
}

// here is my Functional Code

public class FindUserActivity extends AppCompatActivity {
    private RecyclerView mUserList;
    private RecyclerView.Adapter mUserListAdapter;
    private RecyclerView.LayoutManager mUserListLayoutManager;



    ArrayList<UserObject> userList ,contactList;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_find_user);
        contactList = new ArrayList<>();
        userList = new ArrayList<>();

        initializeRecyclerView();
        getContactList();
    }





    private void getContactList(){
        String ISOPrefix = getCountryISO();
        Cursor phones = getContentResolver().query(ContactsContract.CommonDataKinds.Phone.CONTENT_URI,null,null,null,null );
        while (phones.moveToNext()){
            String name = phones.getString(phones.getColumnIndex(ContactsContract.CommonDataKinds.Phone.DISPLAY_NAME));
            String phone = phones.getString(phones.getColumnIndex(ContactsContract.CommonDataKinds.Phone.NUMBER));
            phone = phone.replace(" ","");
            phone = phone.replace("-","");
            phone = phone.replace("(","");
            phone = phone.replace(")","");
            if (!String.valueOf(phone.charAt(0)).equals("+"))
                phone = ISOPrefix + phone;

            UserObject mContact = new UserObject(name,phone);
            userList.add(mContact);
            getUserDetails(mContact);
        }
    }



    @SuppressLint("WrongConstant")
    private void getUserDetails(UserObject mContact) {
        DatabaseReference mUserDB = FirebaseDatabase.getInstance().getReference().child("user");
        Query query = mUserDB.orderByChild("phone").equalTo(mContact.getPhone());
        mUserDB.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot snapshot) {
                if (snapshot.exists()) {
                    String phone = "", name = "";
                    for (DataSnapshot childSnapshot : snapshot.getChildren()) {
                        if (childSnapshot.child("phone").getValue() != null)
                            phone = childSnapshot.child("phone").getValue().toString();

                        if (childSnapshot.child("name").getValue() != null)
                            name = childSnapshot.child("name").getValue().toString();

                        UserObject mUser = new UserObject(name, phone);
                        userList.add(mUser);
                        mUserListAdapter.notifyDataSetChanged();

                        return ;

                    }
                }
            }

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

            }
        });
    }
    private String getCountryISO() {
        String iso = null;

        TelephonyManager telephonyManager = (TelephonyManager) getApplicationContext().getSystemService(getApplicationContext().TELEPHONY_SERVICE);
        if (telephonyManager.getNetworkCountryIso() != null)
            if (!telephonyManager.getNetworkCountryIso().toString().equals(""))
            iso = telephonyManager.getNetworkCountryIso().toString();

        return CountryToPhonePrefix.getPhone(iso);

    }

    @SuppressLint("WrongConstant")
    private void initializeRecyclerView() {
        mUserList= findViewById(R.id.userList);
        mUserList.setNestedScrollingEnabled(false);
        mUserList.setHasFixedSize(false);
        mUserListLayoutManager= new LinearLayoutManager(getApplicationContext(), LinearLayout.VERTICAL,false);
        mUserList.setLayoutManager(mUserListLayoutManager);
        mUserListAdapter= new UserListAdapter(userList);
        mUserList.setAdapter(mUserListAdapter);

    }
}

// This is build.gradle and I call all the dependencies which is neccessary

apply plugin: 'com.android.application'
apply plugin: 'com.google.gms.google-services'

android {
    compileSdkVersion 29
    buildToolsVersion "29.0.3"

    defaultConfig {
        applicationId "com.example.whatsapp"
        minSdkVersion 16
        targetSdkVersion 29
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: "libs", include: ["*.jar"])
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    //noinspection GradleCompatible
    implementation 'com.android.support:design:1.1.0'

    implementation "androidx.recyclerview:recyclerview:1.1.0"
    // For control over item selection of both touch and mouse driven selection
    implementation "androidx.recyclerview:recyclerview-selection:1.1.0-rc01"


    implementation 'com.google.firebase:firebase-core:16.0.5'
    implementation 'com.google.firebase:firebase-auth:16.0.5'
    implementation 'com.google.firebase:firebase-database:16.0.5'
    implementation 'com.firebaseui:firebase-ui-database:4.2.1'

    //implementation 'com.google.firebase:firebase-database:19.3.1'

    //implementation 'com.github.bumptech.glide:glide:4.11.0'
    //annotationProcessor 'com.github.bumptech.glide:compiler:4.11.0'

    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.1'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'

}