WPF Button in DataGrid/ListBox

I've encountered a problem when converting a WPF project from vs2008 to vs2010. I have a DataGrid that contains a ListBox. Each ListBoxItem has a Label and a Button. After converting to vs2010 the button no longer renders but crashes the app as soon as it comes into view. (Ie. the app loads but when the ListBox is created I get a NullReferenceException. What does work though is to remove the click event from the button and then it renders fine :) Same type of setup with Button within ListBoxItem also works when not inside a DataGrid. The content of ListBox obviously is meant to be dynamic but when working with a static collection I get the same error. Also removing the CommandParam does not help at all. Any pointers most welcome.

Code:

<DataGrid x:Name="DgTest" AutoGenerateColumns="false">
        <DataGrid.Columns>
            <DataGridTemplateColumn>
                <DataGridTemplateColumn.CellTemplate>
                    <DataTemplate>
                        <StackPanel>
                            <ListBox ItemsSource="{Binding ItemList}">
                                <ListBox.ItemTemplate>
                                    <DataTemplate >
                                        <StackPanel Style="{StaticResource hzp}">
                                            <Label  />
                                            <Button Click="Button_Click"     Content="TestButton"/>
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                                    </DataTemplate>
                                </ListBox.ItemTemplate>
                            </ListBox>
                        </StackPanel>
                    </DataTemplate>
                </DataGridTemplateColumn.CellTemplate>
            </DataGridTemplateColumn>
        </DataGrid.Columns>
    </DataGrid>

Code-behind:

Imports System.Collections.ObjectModel

Class MainWindow
    Public TestList As New ObservableCollection(Of TestClass)

    Private Sub MainWindow_Loaded(ByVal sender As Object, ByVal e As     System.Windows.RoutedEventArgs) Handles Me.Loaded
    DgTest.ItemsSource = TestList
    TestList.Add(New TestClass(0))
    TestList.Add(New TestClass(1))
    End Sub

    Public Class TestClass
        Private _ItemList As New List(Of String)
        Private _id As Integer

        Public Property ItemList() As List(Of String)
        Get
            Return _ItemList
        End Get
        Set(ByVal value As List(Of String))
            _ItemList = value
        End Set
    End Property
    Public Property Id() As Integer
        Get
            Return _id
        End Get
        Set(ByVal value As Integer)
            _id = value
        End Set
    End Property


    Public Sub New(ByVal id As Integer)
        _ItemList.Add("String1")
        _id = id
    End Sub
End Class

Private Sub Button_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs)

End Sub

End Class

And in App Resources:

<Style TargetType="StackPanel" x:Key="hzp">
        <Setter Property="Orientation" Value="Horizontal"/>
        <Setter Property="Background" Value="Orange"/>
</Style>

Now here's the strange thing. If the Stackpanel Style is removed, the button will work. If the Click event for the button is removed, it will load normally.

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Seems like your event handler is gone from the code-behind file, check that first. Comment if that is not the case.

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I believe I have found the answer to my own question. In the ListBox bound to an ObservableCollection all Styles must be DynamicResource. Using StaticResource worked well in 3.5 but not 4! Took a few hours of randomly testing everything to find this. Case closed