private void AddCharacterButton_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                Character newCharacter = new Character(CharacterLevelSelector.Text, Convert.ToInt32(NumCharactersTextBox.Text));
                characterList.AddToCharacterList(newCharacter);

                //add entry to the character listbox and determine the output string based on number of characters
                ListBoxItem item = new ListBoxItem();
                if (newCharacter.GetNumberOfCharacters() > 1)
                {
                    item.Content = Convert.ToString(newCharacter.GetNumberOfCharacters()) + " Level " + newCharacter.GetCharacterLevel() + " characters";
                }
                else
                {
                    item.Content = Convert.ToString(newCharacter.GetNumberOfCharacters()) + " Level " + newCharacter.GetCharacterLevel() + " character";
                }
                CharacterListBox.Items.Add(item);

                CalculateXPThresholdsButton.IsEnabled = true;

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