Can't retrieve data from firebase (java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String)

this exception appear when I'm trying retrieve user picture (also when I'm trying to get user name or any information about the user.

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.example.signup.Model.User.getProfile_pic()' on a null object reference at com.example.signup.adapter.PostAdapter$1.onDataChange(PostAdapter.java:70) 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$1.run(EventRaiser.java:55) at android.os.Handler.handleCallback(Handler.java:751) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6682) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410)

my code:

     FirebaseDatabase.getInstance().getReference().child("Users").child(post.getPublisher()).addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {

                User user = dataSnapshot.getValue(User.class);
              if (user.getProfile_pic().equals("default")) {
                    holder.imageProfile.setImageResource(R.mipmap.ic_launcher);
                } else {
                    Picasso.get().load(user.getProfile_pic()).placeholder(R.mipmap.ic_launcher).into(holder.imageProfile);
                }

                holder.username.setText(user.getUsername());
            }

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

            }
        });