Problem with retrieving data from FirebaseDatabase

Question

I'm newly working with FirebaseDatabase in android studio. I'm giving you a part of my code that doesn't work but supposed to do.

btnGetInfo.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String str = 33 + "";
            /*
            database = FirebaseDatabase.getInstance();
            reference = database.getReferenceFromUrl("https://test-4e354.firebaseio.com/");
             */
            database.getReference(str).addValueEventListener(
                    new ValueEventListener() {
                        @Override
                        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                            Toast.makeText(MainActivity.this, "test",Toast.LENGTH_LONG)
                                    .show();
                            Boxer boxer = dataSnapshot.getValue(Boxer.class);
                            if(boxer == null) return;

                            txtName.setText(boxer.getName());
                            txtPPower.setText(boxer.getPaunchPower());
                            txtPSpeed.setText(boxer.getPaunchSpeed());
                            txtStamina.setText(boxer.getStamina());
                        }

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

                        }
                    }
            );
        }
    });

The program doesn't even enter in the line

database.getReference(str).addValueEventListener()

When i click on the Button btnGetInfo , my app get crushed. why? I can't find any problem. Can you help me please?

Here is the database structure:

And this is the 'Logcat' :

2020-06-24 19:38:47.534 25686-25686/com.example.firstfirebaseproject E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.firstfirebaseproject, PID: 25686
com.google.firebase.database.DatabaseException: Class com.example.firstfirebaseproject.Boxer does not define a no-argument constructor. If you are using ProGuard, make sure these constructors are not stripped.
    at com.google.firebase.database.core.utilities.encoding.CustomClassMapper$BeanMapper.deserialize(com.google.firebase:firebase-database@@16.0.4:557)
    at com.google.firebase.database.core.utilities.encoding.CustomClassMapper$BeanMapper.deserialize(com.google.firebase:firebase-database@@16.0.4:550)
    at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertBean(com.google.firebase:firebase-database@@16.0.4:420)
    at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.deserializeToClass(com.google.firebase:firebase-database@@16.0.4:214)
    at com.google.firebase.database.core.utilities.encoding.CustomClassMapper.convertToCustomClass(com.google.firebase:firebase-database@@16.0.4:79)
    at com.google.firebase.database.DataSnapshot.getValue(com.google.firebase:firebase-database@@16.0.4:212)
    at com.example.firstfirebaseproject.MainActivity$1$1.onDataChange(MainActivity.java:71)
    at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@16.0.4:75)
    at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@16.0.4:63)
    at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@16.0.4:55)
    at android.os.Handler.handleCallback(Handler.java:873)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:201)
    at android.app.ActivityThread.main(ActivityThread.java:6810)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:547)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:873)

I don't know what does it means.

And this is the Boxer class:

public class Boxer {

private String name, paunchPower, paunchSpeed, stamina;

public Boxer(String name, String paunchPower, String paunchSpeed, String stamina) {
    this.name = name;
    this.paunchPower = paunchPower;
    this.paunchSpeed = paunchSpeed;
    this.stamina = stamina;
}

Thanks <3