ConcurrentModificationException приложение не запускается

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.blum, PID: 12203
    java.util.ConcurrentModificationException
        at java.util.ArrayList$Itr.next(ArrayList.java:860)
        at com.example.blum.Fragments.ChatsFragment$2.onDataChange(ChatsFragment.java:101)
        at com.google.firebase.database.core.ValueEventRegistration.fireEvent(ValueEventRegistration.java:75)
        at com.google.firebase.database.core.view.DataEvent.fire(DataEvent.java:63)
        at com.google.firebase.database.core.view.EventRaiser$1.run(EventRaiser.java:55)
        at android.os.Handler.handleCallback(Handler.java:938)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
I/Process: Sending signal. PID: 12203 SIG: 9

    package com.example.blum.Fragments;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import com.example.blum.Adapter.UserAdapter;
import com.example.blum.Model.Chat;
import com.example.blum.Model.User;
import com.example.blum.R;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;
import java.util.List;

public class ChatsFragment extends Fragment {

    private RecyclerView recyclerView;

    private UserAdapter userAdapter;
    private List<User> mUsers;

    FirebaseUser fuser;
    DatabaseReference reference;

    private List<String> usersList;



    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.fragment_chats, container, false);

        recyclerView = view.findViewById(R.id.recycler_view);
        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));

        fuser = FirebaseAuth.getInstance().getCurrentUser();

        usersList = new ArrayList<>();

        reference = FirebaseDatabase.getInstance().getReference("Chats");
        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                usersList.clear();

                for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                    Chat chat = snapshot.getValue(Chat.class);

                    if (chat.getSender().equals(fuser.getUid())) {
                        usersList.add(chat.getReceiver());
                    }
                    if (chat.getReceiver().equals(fuser.getUid())) {
                        usersList.add(chat.getSender());
                    }
                }

                readChats();
            }

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

            }
        });


        return view;
    }

    private void readChats() {
        mUsers = new ArrayList<>();

        reference = FirebaseDatabase.getInstance().getReference("Users");

        reference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                mUsers.clear();

                for (DataSnapshot snapshot : dataSnapshot.getChildren()) {
                    User user = snapshot.getValue(User.class);

                    for (String id : usersList) {
                        if (user.getId().equals(id)) {
                            if (mUsers.size() != 0 ) {
                                for (User userl : mUsers) {
                                    if (!user.getId().equals(userl.getId())) {
                                        mUsers.add(user);
                                    }
                                }
                            } else {
                                mUsers.add(user);
                            }
                        }
                    }
                }

                userAdapter = new UserAdapter(getContext(), mUsers);
                recyclerView.setAdapter(userAdapter);
            }

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

            }
        });
    }
}