[abenedik]

In the AssimpWpfImporterSample the ContentVisual is defined in XAML:

            <Viewport3D Name="MainViewport">
                <ModelVisual3D x:Name="ContentVisual" />
                <visuals:WireframeVisual3D x:Name="ContentWireframeVisual" ShowPolygonLines="True" WireframeType="Wireframe" UseModelColor="False"
                                           LineColor="Black" LineThickness="1" />                
            </Viewport3D>


I see that you already have a field ContentVisual, but because of the exception, you get it seems that you did not yet create an instance of it. As seen from the XAML, you will need to assign it to a new instance of ModelVisual3D object and then add that instance to Viewport3D objects with (MainViewport is the field that is an instance of Viewport3D):

Private _contentVisual As ModelVisual3D

...

Me._contentVisual = new ModelVisual3D()
Me._viewport3D.Children.Add(_contentVisual)