Properties Panel not working for me

Hello,

I'm trying to write some editor. First I've created a navigator panel, 
which is already working. Now I wanted to also create a properties 
panel, but it doesn't work for me:

I've overridden getSheet() like advised in the platform tutorials, but 
when debugging, the method is never called.

When I'm right-clicking on the root node of my navigator panel, the 
properties of the underlying DataObject's properties are shown instead 
of those of the root node, thought this object is not in the node's lookup.

It seems, instead of the node's properties, the properties of the 
ExplorerManager are shown, so the root node is probably wrapped by it.

How can I fix this problem? And is there any means to show the 
properties panel at the same time my editor window is activated?

Kind regards
Peter