DataTemplateSelector item is Null

Question

This is similar issue to this, but the answer did not work for me.

I have a simple window with a ContentControl. I wanted to use a template selector with it.
I set up the binding for the Content and chose a ContentTemplateSelector (which is located in another file/namespace).

<Window x:Class="EpicNum.View.Floaters.FloaterDnD"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Height="57" Width="130"  
    WindowStyle="None" ResizeMode="NoResize" ShowInTaskbar="False">

    <ContentControl Content="{Binding}"
                    ContentTemplateSelector="{StaticResource NodeTemplateSelector}"
                    />
</Window>

My template selector class looks looks like this:

class NodeTemplateSelector : DataTemplateSelector
{
    public override DataTemplate SelectTemplate(object item, DependencyObject container)
    {
        FrameworkElement UIElement = container as FrameworkElement;

        // Add new information for new Node Template selections here...
        if (item.GetType() == typeof(ViewModel.NodeAdder))
        {
            return UIElement.FindResource("TemplateAdderNode") as DataTemplate;
        }
        else if (item.GetType() == typeof(ViewModel.NodeText))
        {
            return UIElement.FindResource("TemplateTextNode") as DataTemplate;
        }
        return base.SelectTemplate(item, container);
    }
}

Somewhere in the ViewModel, I create a new instance for this window and set the DataContext

FloaterDnD F = new FloaterDnD();
F.DataContext = sourceItem;
F.Show();

Now, for some reason the item in my template selector turns out null. I can't figure out, what am I missing.

At runtime, in code I can see that the DataContext is set correctly

But, when it gets to the template selector, it's null

Might be related.
For some reason, when I set the ContentTemplateSelector in xaml for the window, the UI designer shows an error:


If I remove the assignment for ContentTemplateSelector, then the error is gone and it shows the window as normal.