Fixed unitialized parameter in instances_path stylesheet, leading to
calls to add_instance with no data when passing datatypes insteatd of project
root. That bug was making exceptions (for exemple when editing type) when
having arrays of derivated types in project.

<?xml version='1.0' encoding='utf-8'?>
<BeremizRoot URI_location="LOCAL://">
  <TargetType/>
  <Libraries Enable_Python_Library="true"/>
</BeremizRoot>