This 
is the illegal object pattern in Slate. You’ve used a bad object as a Slate Brush. From the looks of it, you’re feeding the slate brush an image widget. You need to use Texture2D, or MaterialInterface object.
This is the illegal object pattern in Slate. You’ve used a bad object as a Slate Brush. From the looks of it, you’re feeding the slate brush an image widget. You need to use Texture2D, or MaterialInterface object.