Small change to the node space RNA function for opening a node group: pass the node tree as explicit argument plus an optional group node, instead of trying to get the node tree from a node property. This is more flexible for future nodes that want to change the node editor. Node group operators can rely on group node types, but the generic RNA functions should not.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55980 954f8c5b-7b00-dc11-b283-0030488c597c