On two accounts I have characters that have received the Gigantic Cake. The preview has shown nothing. After a few minutes an image will appear in the "place object" line-up, but still with no image if selected, nor in the backpack. I have tried actually placing the Gigantic Cake. It remains invisible, yet the character can run into it...so it is 'solid'...just not visible. I've had these Gigantic Cakes for a couple of days now, and was hoping that the images would load after a time. Alas, still no Cake!
I have the same issue as well but its kind of funny to me. I have the "invisible" cake on my fire and put it in my house. Now there is a spot in my house that blocks your movement but nothing shows to cause it. :D
This is a known issue: 
