Flash Builder stops compiling

Somewhat at random today, Flash Builder stops building/compiling/exporting my project to SWF. Everything appears to be compiling but the date on the SWF file remains unchanged. I think this has happened to me before, and I re-remembered what the issue was. To solve the problem I had to:

  1. Right click the project folder, click Properties
  2. Under Actionscript Applications make sure there is a file listed with a green arrow and/or a blue dot. If not, click “Add..” and add your main class file to this list. It should be automatically set as default.

Everything should work swimmingly now. Not sure what causes this infuriating issue, but the fix is simple enough once you know it.