Editor hangs - caused by EditorSelection

[GlaDOSik]

Operating system or device - Godot version:
Godot 2.1 stable 64 bit, Windows 7

Issue description (what happened, and what was expected):
Editor hangs when you pass node to add_node (from EditorSelection) which isn't in the scene tree. It is a wrong way of using add_node but it shouldn't hang. There should be a check if selected node is inside the tree.

Steps to reproduce:

1. Create an editor plugin
2. Make a new node in script: var newMeshInstance = MeshInstance.new()
3. Try to select this node: get_selection().add_node(newMeshInstance)