I want to add data in firebase like in facebook search a person and then add then as your friend

Here is my database structure. First i want to search a person in fire-base database and then add then in logged-in account as a child under "FriendList" node.

The query which i implemented gives me correct result and it also add the value as child in the logedin account but when i want to add the second person in "Friendlist" node or want to search the data the app get crashes and give NullPointerException error. I have added my search code.

    public void SearchData(final String search, final int flag){

        if(flag==1){

            arfullname.clear();
            aridkey.clear();
            arpassword.clear();
            arfirstname.clear();
            aremail.clear();
            arlastname.clear();
        }
        Query query = mDatabase.orderByChild("fullname");

        query.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {


                for (DataSnapshot childd:dataSnapshot.getChildren()){

                    Log.d("TAG123",""+childd.child("fullname").getValue().toString().equals(search));


                        if(childd.child("fullname").getValue().toString().contains(search) ){

                            firstname=childd.child("firstname").getValue().toString();
                            lastname=childd.child("lastname").getValue().toString();
                            email=childd.child("email").getValue().toString();
                            fullname=childd.child("fullname").getValue().toString();
                            password=childd.child("password").getValue().toString();
                            idkey=childd.child("key").getValue().toString();
                            arfirstname.add(firstname);
                            arlastname.add(lastname);
                            aremail.add(email);
                            arfullname.add(fullname);
                            arpassword.add(password);
                            aridkey.add(idkey);

                            Log.d("customdata",firstname+"}"+lastname+"}"+fullname+"}"+idkey);
                            personArrayListModel=new PersonArrayListModel(arfullname,arfirstname,arlastname,aridkey,aremail,arpassword);
                            check=true;

                        }

                        else
                        {

                            Toast.makeText(MainActivity.this, "Not Found", Toast.LENGTH_SHORT).show();

                        }

                    }


                if(check==true && flag==1){

                    ArrayList<CustomFirebaseClass> customFirebaseClasses=getData();
                    CustomFirebaseAdapter customFirebaseAdapter=new CustomFirebaseAdapter(getApplicationContext(),0,customFirebaseClasses);
                    listView.setAdapter(customFirebaseAdapter);
                    Log.d("person123",personArrayListModel.getFullname().toString());

                }
                if(flag==2){

                    Toast.makeText(MainActivity.this, "222", Toast.LENGTH_SHORT).show();
                    Log.d("persondata",firstname+""+lastname+""+fullname+""+email+""+password+""+idkey);
                    User userr = new User(firstname,lastname,fullname,email,password,idkey);

                    try{

                        databaseReference.child(mAuth.getUid()).child("FirendsList").child(idkey).setValue(userr);

                    }catch (RuntimeException e){

                        Toast.makeText(getApplicationContext(), ""+e.getMessage(), Toast.LENGTH_SHORT).show();
                    }

                }

            }

            @Override
            public void onChildChanged(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

            }

            @Override
            public void onChildRemoved(@NonNull DataSnapshot dataSnapshot) {

            }

            @Override
            public void onChildMoved(@NonNull DataSnapshot dataSnapshot, @Nullable String s) {

            }

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

            }
        });


    }