返回导航时出现NullReferenceException

我正在开发Windows Phone 8+应用程序。我有一个页面，其中包含项目的详细信息。每个Item都可以是另一个Item的一个组件，因此我有两个ListBox，其中包含可以组成或组成当前Item的其他Item。

        JItem.Item currItem;
        List<JItem.Item> components;
        List<JItem.Item> isComponentOf;
        protected override void OnNavigatedTo(NavigationEventArgs e)
        {
            base.OnNavigatedTo(e);
            currItemId = NavigationContext.QueryString["item_id"];
            currItem = (JItem.Item)App.itemsDict[currItemId];
            LayoutRoot.DataContext = currItem;
            components = new List<JItem.Item>();
            isComponentOf = new List<JItem.Item>();
            foreach (var x in currItem.components)
            {
                components.Add((JItem.Item)App.itemsDict[x]);
            }
            foreach (var x in currItem.composes)
            {
                isComponentOf.Add((JItem.Item)App.itemsDict[x]);
            }
            componentsLB.ItemsSource = components;
            composesLB.ItemsSource = isComponentOf;
            if (components.Count == 0) 
                componentsLabel.Visibility=Visibility.Collapsed;
            if (isComponentOf.Count == 0) 
                isComponentLabel.Visibility=Visibility.Collapsed;
        }

如果我在componentsLB列表框或isComponentOf列表框中向前导航选择一个项目，一切都很好。如果我通过后退按钮返回到某个页面，即使components包含项目并且分配了componentsLB，我也会在componentsLB.ItemsSource = components;行得到一个System.NullReferenceException。

由于它在NavigationStack上，所以页面不会被重新初始化（构造函数中的InitializeComponent()没有被调用），而是分配了ListBox。

当通过Back导航访问页面时，我可以使用一些变通方法（例如不重新加载数据），但我想了解是什么导致了错误。

这是出现气泡的异常的Stacktrace：

   at DOTA2_Pocket.ItemDetails.componentsLB_SelectionChanged(Object sender, SelectionChangedEventArgs e)
   at System.Windows.Controls.Primitives.Selector.OnSelectionChanged(SelectionChangedEventArgs e)
   at System.Windows.Controls.Primitives.Selector.InvokeSelectionChanged(List`1 unselectedItems, List`1 selectedItems)
   at System.Windows.Controls.Primitives.Selector.SelectionChanger.End()
   at System.Windows.Controls.Primitives.Selector.OnItemsChanged(NotifyCollectionChangedEventArgs e)
   at System.Windows.Controls.ListBox.OnItemsChanged(NotifyCollectionChangedEventArgs e)
   at System.Windows.Controls.ItemsControl.OnItemCollectionChanged(Object sender, NotifyCollectionChangedEventArgs e)
   at System.Collections.Specialized.NotifyCollectionChangedEventHandler.Invoke(Object sender, NotifyCollectionChangedEventArgs e)
   at System.Windows.Controls.ItemCollection.NotifyCollectionChanged(NotifyCollectionChangedEventArgs e)
   at System.Windows.Controls.ItemCollection.UpdateItemsSourceList(IEnumerable newItemsSource)
   at System.Windows.Controls.ItemsControl.ItemsSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
   at System.Windows.DependencyObject.RaisePropertyChangeNotifications(DependencyProperty dp, Object oldValue, Object newValue)
   at System.Windows.DependencyObject.UpdateEffectiveValue(DependencyProperty property, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, ValueOperation operation)
   at System.Windows.DependencyObject.SetValueInternal(DependencyProperty dp, Object value, Boolean allowReadOnlySet)
   at System.Windows.Controls.ItemsControl.set_ItemsSource(IEnumerable value)
   at DOTA2_Pocket.ItemDetails.OnNavigatedTo(NavigationEventArgs e)
   at Microsoft.Phone.Controls.PhoneApplicationPage.InternalOnNavigatedTo(NavigationEventArgs e)
   at Microsoft.Phone.Controls.PhoneApplicationPage.Microsoft.Phone.Controls.IPhoneApplicationPage.InternalOnNavigatedToX(NavigationEventArgs e)
   at System.Windows.Navigation.NavigationService.RaiseNavigated(Object content, Uri uri, NavigationMode mode, Boolean isNavigationInitiator, IPhoneApplicationPage existingContentPage, IPhoneApplicationPage newContentPage)
   at System.Windows.Navigation.NavigationService.CompleteNavigation(DependencyObject content, NavigationMode mode)
   at System.Windows.Navigation.NavigationService.<>c__DisplayClass7.<NavigateCore_StartNavigation>b__4()