I code in Java in Intellij. Everything works fine. But sometimes, all on a sudden there appear a problem. When I try to run a code ( which has been run before ), suddenly build starts failing. It give me this error: "Module 'NewSample' production: java.lang.NullPointerException: Null elements are not allowed in THashSet " After that, whatever I do, this project doesn't work anymore. I have to create a new project. Then everything works fine again until this problem happens in this new project too.

Can somebody give me a solution, or explain why this happens?