Plugin Development :how to get the path of opened project while running plugin

[magbroxwt]

I am devloping my plugin on netbeans platform which need to get .json from currently opened project.
I find some solution on Internet, but all failed.As following:
https://stackoverflow.com/questions/9428903/correct-way-to-get-the-file-path-of-the-currently-opened-project-in-netbeans/16241955
code :

Lookup lookup = Utilities.actionsGlobalContext(); 
Project project = lookup.lookup(Project.class); 
FileObject projectDir = project.getProjectDirectory();  
String projectLocation = projectDir.getPath();

Then , I tried :

TopComponent activeC = TopComponent.getRegistry().getActivated();
FileObject dataLookup = activeC.getLookup().lookup(FileObject.class);
if (dataLookup != null) {
    Project pro = FileOwnerQuery.getOwner(dataLookup);
    path = pro.getProjectDirectory().getPath();
}

Summary : project always turn to be null, and dataLookup too. I wonder if there is another way to get the path of opened project by user. For example，user will install my plugin and it will try to get .json under the root folder of user's local project.
Thanks for any help.