OK... I think I can re-create the bug. What I did, is I accidentally drag the image file into the scene area. And it created an actor for me. And when I deleted it by pressing DEL key, nothing happens. And I use the "minus sign" to delete. And then, I click the "plus sign" in the image area to add an image, somehow, all my image cannot be displayed. And the proj file got corrupted again!
Hmmm... I believe I did the same mistake yesterday... later got time, I will verify. Luckily this time, I never press the SAVE button to save this corrupted file.
If you face with this, simply kill Game salad and restart again.