ProjectManager in JUnit tests

I'm trying to open my custom project programmatically in JUnit test. But when I'm doing this:
[code]
ProjectManager manager = ProjectManager.getDefault();
[/code]
an error occurs:
[i]
java.lang.ExceptionInInitializerError: null
	at org.netbeans.api.project.ProjectManager.<init>(ProjectManager.java:76)
	at org.netbeans.api.project.ProjectManager.<clinit>(ProjectManager.java:70)
[/i]
I have no idea why it happens. Could you give me some hints on how to do this?

Thank you in advance.