"!id_map.has(p_rid.get_data())" error with YSort populating during runtime

[TheFlamyy]

Godot version: 03bd4d2

OS/device including version: Ubuntu

Issue description:
Creating and adding Node2Ds to a YSort after setting up the tree, will result in a spam in the debugger with the following error:

  <C Source>     ./core/rid.h:164 @ getornull()
  <Stack Trace>  Player.gd:31 @ _physics_process()

(The last line is just my debug script)

The following image should illustrate my issue/concern:

Steps to reproduce:
Create a new scene with YSort as the base node.
Include add_child(Node2D.new()) in any function which is called after the tree is initialized (e.g. not ready)
Look at the errors in the debugger.

[bojidar-bg]

Will look into it during the weekend (or a bit after it), sorry for the delay so far.