I'll consider that, thanks for the suggestion!
However, note that the AssetManager actually doesn't return a white texture, but simply "null" when you ask it for an object it does not contain. The reason why you're seeing a white square is that you're passing that "null" texture to the Image constructor. That will create an image without a texture – hence, white. 🙂
(And this also gives you a chance to check for 'null' yourself when querying the texture.)