java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.adarsh.buddiesgram.Model.User.getImageUrl()' on a null object

Question

FirebaseDatabase.getInstance().getReference().child("Users").child(post.getPublisher()).addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(@NonNull DataSnapshot snapshot) {
        User mainUser = snapshot.getValue(User.class);
        if (mainUser.getImageUrl().equals("default")){
            holder.imageProfile.setImageResource(R.mipmap.ic_launcher);
        } else {
            Picasso.get().load(mainUser.getImageUrl()).into(holder.imageProfile);
        }
        holder.username.setText(mainUser.getUsername());
        holder.author.setText(mainUser.getName());
    }

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

Error:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.adarsh.buddiesgram, PID: 21987
    java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.adarsh.buddiesgram.Model.User.getImageUrl()' on a null object reference
        at com.adarsh.buddiesgram.Adapter.PostAdapter$1.onDataChange(PostAdapter.java:68)
        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$1.run(EventRaiser.java:55)
        at android.os.Handler.handleCallback(Handler.java:790)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)