Allright, I think I fixed it but I don’t know why 
I deleted the method
[java]nifty.validateXml(“src/org/jplane/gui/start.xml”);[/java]
and put the following lines from the nifty tutorial into my xml file:
[xml]<nifty xmlns=“http://nifty-gui.sourceforge.net/nifty.xsd”
xmlns:xsi=“http://www.w3.org/2001/XMLSchema-instance”
xsi:schemaLocation=“http://nifty-gui.sourceforge.net/nifty.xsd
http://nifty-gui.sourceforge.net/nifty.xsd”>
[/xml]
It still says that there is a missing screencontroller for screen [null], but that doesn’t matter, if I click the label, the method is invoked.
However, I used the element panel again and not control, I think I need to install an add-on first.
Again, thank you for working on that problem.
I would refer to a blog post that explains the XML validation but that is an old post and for Nifty 1.3 you should use a different xsd url. But that is even more complicated and so I’ll just keep my mouth shut to not confuse you any further… 
I find why it`s not working:
1.screenController class must be have an empty constructor method
2. in nifty xml file and in screen tag you must use Lowercase letter for first letter of controller=“package.test” not Controller=""