You are correct in your assumption that Stencyl created the 50-0.png file. It's supposed to be an animation for one of your actors, probably one that you deleted. Here's how this problem can usually be solved:
- In Stencyl, press
Debug > View > View Folder for Your Game, then open
.../YourGameName /resources.
- Inside this folder should be the images for all of your actors, backgrounds and so on. Create an image called
50-0.png; it shouldn't matter what it is, so it can be blank.
I hope this helps you solve the problem. Happy Easter!