Error retrieving Application Engine Component

After creating a brand new App Engine in a dev environment and getting it running successfully in the dev environment, I received the following error when it was migrated to test:

Error retrieving Application Engine (ProgramName).MAIN component: Step Step01, PeopleCode

I've seen errors like this before. Usually the reason this happens is a step on the App Engine wasn't included in the project. In Tools, I reinserted all the definitions for the app engine and migrated it again only to get the same error.

Next I replaced the entire Peoplecode section in the app engine with this:

Exit(0);

The program finished successfully in test but after replacing the exit statement with the Peoplecode that actually does something, I got the error again. This was strange to me because the error indicated that there was a structural problem with the App Engine.

The only thing left to do was double check the app package imports. After reinserting all of the application package definitions (and sub definitions) and moving the program up, the program worked fine.

This kind of problem can occur if new definitions aren't saved to the project because of a Peopletools crash or something similar. For example, a new app package is created and the program and project are saved. Next, a new class is created and saved and Tools is shutdown without saving the project. The class still exists in the dev environment but it isn't part of the project.