It happened the same to me. It seems that we were using the old phpunit-skelgen 1.2.1 version. Apparently, NetBeans 8.0.1 assumes you are using the newest version of phpunit-skelgen, that is 2.0.1.
So, you only have to update the phpunit-skelgen version, for example using this: https://phar.phpunit.de/phpunit-skelgen.phar
I copied that file to where I had my previous phpunit-skelgen.bat files: C:\xampp\php. Of course, this may vary according to your OS or your project structure.
After that, configure NetBeans to use the downloaded file instead of the old file. Something around Tools > Options in Netbeans menu.
Hope this helps!