C# problem z Element is already the child of another element.

Question

Cześć, jestem w trakcie nauki pisania w C#, szło wszystko gładko aż w pewnym momencie kiedy chcę uruchomić swoje wypociny wyskakuje mi taki alert :

" An exception of type 'System.Exception' occurred in mscorlib.dll but was not handled in user code

WinRT information: Element is already the child of another element.

Additional information: No installed components were detected.

Element is already the child of another element. "

Nie mam pojęcia co zrobiłem źle i już mam dosyć studiowania ciągle tego kodu w poszukiwaniu błędu. Osobiście uważam, że program wali się przy "playArea.Children.Add" bo po usunięciu tych 3 linijek program działa ale są mi one potrzebne.

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;

using Windows.UI.Xaml.Media.Animation;

// The Basic Page item template is documented at http://go.microsoft.com/fwlink/?LinkId=234237

namespace Ratuj_ludzi
{
    /// <summary>
    /// A basic page that provides characteristics common to most applications.
    /// </summary>
    public sealed partial class MainPage : Ratuj_ludzi.Common.LayoutAwarePage
    {
        Random random = new Random();
        DispatcherTimer enemyTimer = new DispatcherTimer();
        DispatcherTimer targetTimer = new DispatcherTimer();
        bool humanCaptured = false;

        public MainPage()
        {
            this.InitializeComponent();

            enemyTimer.Tick += EnemyTimer_Tick;
            enemyTimer.Interval = TimeSpan.FromSeconds(2);

            targetTimer.Tick += TargetTimer_Tick;
            targetTimer.Interval = TimeSpan.FromSeconds(1);

        
        }

        private void TargetTimer_Tick(object sender, object e)
        {
            progressBar.Value += 1;
            if (progressBar.Value >= progressBar.Maximum)
                EndTheGame();
        }

        private void EndTheGame()
        {
            if (!playArea.Children.Contains(gameOverText))
            {
                enemyTimer.Stop();
                targetTimer.Stop();
                humanCaptured = false;
                startButton.Visibility = Visibility.Visible;
                playArea.Children.Add(gameOverText);
            
            }
        }

        private void EnemyTimer_Tick(object sender, object e)
        {
            AddEnemy();
        }

        /// <summary>
        /// Populates the page with content passed during navigation.  Any saved state is also
        /// provided when recreating a page from a prior session.
        /// </summary>
        /// <param name="navigationParameter">The parameter value passed to
        /// <see cref="Frame.Navigate(Type, Object)"/> when this page was initially requested.
        /// </param>
        /// <param name="pageState">A dictionary of state preserved by this page during an earlier
        /// session.  This will be null the first time a page is visited.</param>
        protected override void LoadState(Object navigationParameter, Dictionary<String, Object> pageState)
        {
        }

        /// <summary>
        /// Preserves state associated with this page in case the application is suspended or the
        /// page is discarded from the navigation cache.  Values must conform to the serialization
        /// requirements of <see cref="SuspensionManager.SessionState"/>.
        /// </summary>
        /// <param name="pageState">An empty dictionary to be populated with serializable state.</param>
        protected override void SaveState(Dictionary<String, Object> pageState)
        {
        }

        private void startButton_Click(object sender, RoutedEventArgs e)
        {
            StartGame();
        }

        private void StartGame()
        {
            human.IsHitTestVisible = true;
            humanCaptured = false;
            progressBar.Value = 0;
            startButton.Visibility = Visibility.Collapsed;
            playArea.Children.Clear();
            playArea.Children.Add(target);
            playArea.Children.Add(human);
            enemyTimer.Start();
            targetTimer.Start();
        }

        private void AddEnemy()
        {
            ContentControl enemy = new ContentControl();
            enemy.Template = Resources["EnemyTemplate"] as ControlTemplate;
            AnimateEnemy(enemy, 0, playArea.ActualWidth - 100, "(Canvas.Left)");
            AnimateEnemy(enemy, random.Next((int)playArea.ActualHeight - 100),
                random.Next((int)playArea.ActualHeight - 100), "(Canvas.Top)");
            playArea.Children.Add(enemy);
        }

        private void AnimateEnemy(ContentControl enemy, double from, double to, string propertyToAnimate)
        {
            Storyboard storyboard = new Storyboard() {AutoReverse = true, RepeatBehavior = RepeatBehavior.Forever };
            DoubleAnimation animation = new DoubleAnimation()
            {
                From = from,
                To = to,
                Duration = new Duration(TimeSpan.FromSeconds(random.Next(4, 6)))
            };
            Storyboard.SetTarget(animation, enemy);
            Storyboard.SetTargetProperty(animation, propertyToAnimate);
            storyboard.Children.Add(animation);
            storyboard.Begin();
        }
    }
    }