NullPointerException on struts initialization

When migrate from 2.1.8.1 to 2.2.1 the same project fails in initialization, in the convention plugin PackageBasedconfigBuilder method buildconfiguration line 592 it seems the method getPackge() return null. I solve the problem replace convention-plugin 2.2.1 for 2.1.8.1.

I dont make extensive test but works fine replacing the library.