Android Studio, Firebase – List error

I’m trying to get an author of post from List and I’m getting error:

java.lang.ArrayIndexOutOfBoundsException: length=12; index=-1

Program always gives me index which equals -1
Here is the code from which it gets error:

@Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {

            if(dataSnapshot.hasChild(mCurrentUser.getUid())){

                mDatabasePosts.orderByKey().equalTo(dataSnapshot.getKey()).addValueEventListener(new ValueEventListener() {
                    @Override
                    public void onDataChange(DataSnapshot dataSnapshot) {
                        if(dataSnapshot.exists()) {

                            if(mPostModelList.size() == 1) {
                                mPostModelList.clear();
                                mAdapter.notifyDataSetChanged();
                            }else if(mPostModelList.size() > 1){
                                Post post = dataSnapshot.getValue(Post.class);

                                //mPostModelList.remove(post);

                                /* ERROR -----> */ String p = mPostModelList.get(mPostModelList.indexOf(post)).getDesc(); // <----- ERROR

                                Toast.makeText(mView.getContext(), p, Toast.LENGTH_SHORT).show();


                                mAdapter.notifyDataSetChanged();
                            }
                        }
                    }

                    @Override
                    public void onCancelled(DatabaseError databaseError) {

                    }
                });