It’s asserting because your wrapper CDO (Class Default Object) is trying to use Slate before it’s been initialized. This will be different in a packaged game since they’re monolithic, whereas in the editor you’re getting away with it because Slate is initialized before your game module loads.
The simplest fix would be to avoid creating the actual Slate widget for the CDO instance. You can do this by checking whether the object has the RF_ClassDefaultObject flag set.