WPF - VisualBrush Binding

Question

I have a simple control that I've created. In the control's template, there is a rectangle whose VisualBrush's Visual is bound to a dependency property in the control.

I instantiate the control from within a UserControl in my app, and bind the dependency property to an element in my UserControl. The objective is to give a "preview" of some content. I have included the XAML code below to illustrate what I am doing. Anyway, when I do this in my test project, everything wires up correctly. However, when I do this in another project, I hit an exception in a block of code in my control, stating that the dependency property that I bound to an element in my UserControl is NULL. This has me baffled.

Below is the code for my Control Template:

<ControlTemplate x:Key="PanZoomTemplate" TargetType="{x:Type Controls:PanZoom}">
    <Grid DataContext="{Binding Path=View, RelativeSource={RelativeSource TemplatedParent}}" >
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <Grid Grid.Row="0" Height="225" Width="202" x:Name="PART_Content" Background="Green" VerticalAlignment="Stretch" HorizontalAlignment="Left">
            <Canvas>
                <Rectangle x:Name="PART_Visual"  Height="{Binding Path=Children[0].ActualHeight}" Width="{Binding Path=Children[0].ActualWidth}" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
                    <Rectangle.Fill>
                        <VisualBrush Visual="{Binding}"/>
                    </Rectangle.Fill>
                </Rectangle>
                <Thumb x:Name="PART_Thumb" IsHitTestVisible="True" HorizontalAlignment="Left" VerticalAlignment="Top"
                Canvas.Left="{Binding Path=ContentOffsetX, RelativeSource={RelativeSource TemplatedParent},Mode=TwoWay}"
                Canvas.Top="{Binding Path=ContentOffsetY, RelativeSource={RelativeSource TemplatedParent},Mode=TwoWay}"
                Width="{Binding Path=ThumbWidth, RelativeSource={RelativeSource TemplatedParent},Mode=TwoWay}"
                Height="{Binding Path=ThumbHeight, RelativeSource={RelativeSource TemplatedParent},Mode=TwoWay}"
                Opacity="0.3"
                >
                    <Thumb.Template>
                        <ControlTemplate 
                        TargetType="{x:Type Thumb}"
                        >

                            <Border 
                            BorderBrush="Black"
                            BorderThickness="1"
                            Background="Blue"
                            CornerRadius="1"                                        
                            />
                        </ControlTemplate>
                    </Thumb.Template>
                </Thumb>
            </Canvas>
        </Grid>
        <Grid Grid.Row="1" Height="40" Width="200" HorizontalAlignment="Left" VerticalAlignment="Bottom" Background="Transparent">
            <Slider
    Style="{StaticResource SliderStyle1}"
    Minimum="10"
    Maximum="250"
    Height="28" Value="{Binding Path=ScaleFactor, RelativeSource={RelativeSource TemplatedParent},Mode=TwoWay}"
                behaviors:SliderValueChange.Command="{Binding Path=ZoomInCommand, RelativeSource={RelativeSource TemplatedParent}}"
    HorizontalAlignment="Left" 
    VerticalAlignment="Top"
    Width="130"
    Orientation="Horizontal"/>
        </Grid>
    </Grid>
</ControlTemplate>

Here is the code in my UserControl, that instantiates the control:

<controls:PanZoom x:Name="PanZoomControl" Template="{StaticResource PanZoomTemplate}"  View="{Binding ElementName=ContentPane, Path=SelectedContent}"/>

The element ContentPane is a TabControl in the UserControl.

Lastly, in the Loaded event handler of my control, I am trying to reference the ActualHeight and ActualWidth properties of my View dependency property, that should have been bound already to the ContentPane element; however I hit a null exception:

_unScaledExtent = new Size(View.ActualWidth, View.ActualHeight);

So, in my test project, works great, but in another project with identical code, except the TabControl is being populated with different content, it hits an exception. Thoughts??? What am I doing incorrectly???