SL 5打破了我的组合框

我有以下xaml在SL 4中工作正常，但在SL 5插件下运行时似乎被打破了。

I have the following xaml that works fine in SL 4, but seems to be broken when run under SL 5 plug-in.

            <StackPanel 
               Orientation="Horizontal"                        
               Grid.Column="1" 
               Grid.Row="3" 
               Margin="2"
               Visibility="{Binding HasCategory1, Converter={StaticResource BooleanToVisibility}}"
               >
               
               <ComboBox 
                  x:Name="Category1Combo"
                  DataContext="{Binding Categories}"
                  ItemsSource= "{Binding Category1Choices}" 
                  SelectedItem="{Binding SelectedCategory1, Mode=TwoWay, NotifyOnValidationError=True, ValidatesOnDataErrors=True, ValidatesOnExceptions=True }"
                  DisplayMemberPath="Name" 
                  />

               <ComboBox 
                  DataContext="{Binding Categories}"
                  ItemsSource= "{Binding SubCategory1List }" 
                  SelectedItem="{Binding SelectedSubCategory1, Mode=TwoWay}"
                  DisplayMemberPath="Name" 
                  Margin="5,0,0,0"
                  Visibility="{Binding SelectedItem, ElementName=Category1Combo, Converter={StaticResource HasSubCategoriesToVisibility}}"
                  >
               </ComboBox>
            </StackPanel>

            <StackPanel 
               Orientation="Horizontal"                        
               Grid.Column="1" 
               Grid.Row="4" 
               Margin="2"
               Visibility="{Binding HasCategory2, Converter={StaticResource BooleanToVisibility}}"
               >
               <ComboBox 
                  x:Name="Category2Combo" 
                  DataContext= "{Binding Categories}"
                  ItemsSource= "{Binding Category2Choices}" 
                  SelectedItem="{Binding SelectedCategory2, Mode=TwoWay, NotifyOnValidationError=True, ValidatesOnDataErrors=True, ValidatesOnExceptions=True }"
                  DisplayMemberPath="Name"
                  />
               <ComboBox 
                  DataContext= "{Binding Categories}"
                  ItemsSource= "{Binding SubCategory2List }" 
                  SelectedItem="{Binding SelectedSubCategory2, Mode=TwoWay}"
                  DisplayMemberPath="Name" 
                  Margin="5,0,0,0"
                  Visibility="{Binding SelectedItem, ElementName=Category2Combo, Converter={StaticResource HasSubCategoriesToVisibility}}"
                  />
            </StackPanel>

在两个StackPanel中，ItemSource和SelectedItem在加载过程中都设置为值。 没有值为null。 当第二对组合框设置了SelectedSubCategory2时，我在SL 5下运行时出错。

In both StackPanels, the ItemSource and SelectedItem are set to values during the load process.  No values are null.  When the second pair of comboboxes has its SelectedSubCategory2 set, I get an error when running under SL 5.

错误说：

Unhandled Error in Silverlight Application 
Code: 4004    
Category: ManagedRuntimeError       
Message: System.ArgumentException: GeneratorPosition '-1,1' passed to Remove does not have Offset equal to 0.

Parameter name: position

   at System.Windows.Controls.ItemContainerGenerator.Remove(GeneratorPosition position, Int32 count, Boolean isRecycling)

   at System.Windows.Controls.ItemContainerGenerator.System.Windows.Controls.Primitives.IItemContainerGenerator.Remove(GeneratorPosition position, Int32 count)

   at System.Windows.Controls.ComboBox.SetContentPresenter(Int32 index)

   at System.Windows.Controls.ComboBox.OnApplyTemplate()

   at System.Windows.FrameworkElement.OnApplyTemplate(IntPtr nativeTarget)  

任何人都可以告诉我它抱怨什么以及如何解决这个问题？ 

Can anybody give me a clue as to what it is complaining about and how to get around this problem?