Problem binding DataGridComboBoxColumn.ItemsSource

I have 3 tables: Item - which is the DataContext - it has a navigation column Group Group - has a navigation column Category.

I want to have in the DataGrid both (Category & Group) columns and when I choose a category it should display in the group col only the Category.Groups.

Here is the code I am working on:

<tk:DataGrid AutoGenerateColumns="False" ItemsSource="{Binding}">
    <tk:DataGrid.Columns>

        <!--Works-->
        <tk:DataGridComboBoxColumn                                        
            Header="Categroy" 
            DisplayMemberPath="Title"                    
            SelectedValuePath="CategoryId"
            SelectedValueBinding="{Binding Group.Category.CategoryId}"
            ItemsSource="{Binding Context.Categories, 
                Source={x:Static Application.Current}}"
        />


        <!--Look at these two things:-->

        <!--This does work-->
        <tk:DataGridTemplateColumn>
            <tk:DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                    <ItemsControl
                        ItemsSource="{Binding Group.Category.Groups}">
                        <ItemsControl.ItemTemplate>
                            <DataTemplate DataType="{x:Type data:Group}">
                                <TextBlock Text="{Binding Title}"/>
                            </DataTemplate>
                        </ItemsControl.ItemTemplate>
                    </ItemsControl>
                </DataTemplate>
            </tk:DataGridTemplateColumn.CellTemplate>
        </tk:DataGridTemplateColumn>

        <!--But this does NOT work, even it's the same source-->
        <!--Notice I even tried a dummy converter and doesnt reach there-->
        <tk:DataGridComboBoxColumn 
            Header="Group" 
            DisplayMemberPath="Title"