Adding a child to Viewport3D asynchronously gives "This API was accessed with arguments from the wrong context."

Question

When I try to add 3D-content to a Viewport3D, asynchronously, this results in "This API was accessed with arguments from the wrong context." in a TargetInvocationException.

The 3D-content is generated from the data of a 3D-scanning device. The communication&calculations needed for that are done in a separate thread. First, I tried to acces the viewport3D from that thread. I realized this should be done by the GUI-thread, so now I use this code:

        ModelVisual3D model = new ModelVisual3D();
        model.Content = scanline;

        DispatcherOperation dispOp = this.viewport.Dispatcher.BeginInvoke(
            new AddModelDelegate(StartAddModel), model);
    }
    private void StartAddModel(ModelVisual3D model)
    {
        this.viewport.Children.Add(model); 
        //model is not in the context of this current thread. 
        //Throws exception: "This API was accessed with arguments from the wrong context."
    }

    private delegate void AddModelDelegate(ModelVisual3D model);

It seems that the object named "model" is not in the context of the current thread. How can I fix this? Is there a way to get the model to the context of the Dispatcher? Or is this way of doing this just not the way to go here?