Cannot save settings: Root already belongs to main

Hi. I've started an open-source project some time ago with IntelliJ IDEA (Community Edition) and uploaded it to GitHub. My previous computer died and I had to reinstate the project from my GitHub repository and import the project into new (because GitHub does not have project files, just the source), and again after I had to clean install Windows again. I'm trying to remove the jetbrains dependency on NotNull import by a more generic, but it seems to be outside Java language scope :(

I'm trying to import this class for annotations (I want to remove JetBrains depencency, because if someone uses different IDE, it will produce error)

import javax.validation.constraints.NotNull;

But I've encountered this issue. That class isn't in standard Java library, it needs to be imported from external JAR dependency. And if I do so, it will fail:

How do I import it? Why does this show me that error?