DoubleClick on the ToolWindow causes NullPointerException if no nodes are selected in the Tree

Hi,

first of all thank you for the great project, I had also planned to create a plugin for zookeeper, but you were faster ;)

Anyhow I detected a small bug: after double-clicking on the ToolWindow (by mistake) I got a nasty NullPointerException in ZkProjectComponent.onDoubleClick, because no nodes were selected.

Regards,
Nandor