Trying to save scene cause viewport_set_canvas_stacking: Condition ' !viewport ' is true.

[qarmin]

Godot version:
3.1 66c6dfb

OS/device including version:
Windows 10

Issue description:
When I save scene from below then this error is shown:

ERROR: viewport_set_canvas_stacking: Condition ' !viewport ' is true.
   At: servers/visual/visual_server_viewport.cpp:541

Steps to reproduce:

1. Copy code from below and save as Scene.tscn
2. Open this scene
3. Save this scene

Minimal reproduction project:

[gd_scene format=2]

[node name="Game" type="Node2D"]

[node name="Camera" type="Camera2D" parent="."]

[node name="HUD" type="CanvasLayer" parent="."]

[akien-mga]

CC @RandomShaper

[RandomShaper]

Oh, I think I now what's happening. I'll fix it (as well as the 3.0 PR) in a few hours.