How to ignore the "project jdk is not defined" warning?

I am developing jdk source, so I dont wanto set external jdk in project.Because if the jdk is set, then IDEA will report unexpected errors, and I don't need to rely on an external jdk to check the code. Now the problem is that if I don't set the jdk, then there is a warning bar that always appears on the interface, and I can't find any button to close it.

The waring bar is like this: