private void AddMonsterButton_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                Monster newMonster = new Monster(MonsterCRSelector.Text, Convert.ToInt32(NumMonstersTextBox.Text));
                monsterList.AddtoMonsterList(newMonster);

                //add entry to the monster listbox and determine the output string based on number of monsters
                ListBoxItem item = new ListBoxItem();
                if (newMonster.GetNumberOfMonsters() > 1)
                {
                    item.Content = Convert.ToString(newMonster.GetNumberOfMonsters()) + " monsters with a CR of " + newMonster.GetChallengeRating();
                }
                else
                {
                    item.Content = Convert.ToString(newMonster.GetNumberOfMonsters()) + " monster with a CR of " + newMonster.GetChallengeRating();
                }
                MonsterListBox.Items.Add(item);

                //determine if CalculateXPButton should be enabled
                if (AllListsAreNotEmpty(characterList, monsterList))
                {
                    CalculateXPButton.IsEnabled        = true;
                    AddToEncounterListButton.IsEnabled = true;
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }