Adding Gridmap node outputs multiple error messages

[Pandazuechter]

Godot version: v3.0.2.stable.official

OS/device including version: Windows 10

Issue description:
Related to #15515 but no undo, camera etc.

Adding a Gridmap to a scene generates multiple error messages .

Steps to reproduce:
New scene -> Spatial -> Gridmap
Save scene -> run it -> stop it

Error messages on play if gridmap node was selected:
c:\projects\godot-builds\godot\core\rid.h:154 - Condition ' !p_rid.is_valid() ' is true. returned: 0 servers\visual\visual_server_scene.cpp:620 - Condition ' !instance ' is true. core\object.cpp:1025 - Condition ' !metadata.has(p_name) ' is true. returned: Variant()

Clicking on the gridmap node yields this error (not during play):
core\object.cpp:1025 - Condition ' !metadata.has(p_name) ' is true. returned: Variant()

Clicking on the spatial node outputs these errors (not during play):
c:\projects\godot-builds\godot\core\rid.h:154 - Condition ' !p_rid.is_valid() ' is true. returned: 0 servers\visual\visual_server_scene.cpp:620 - Condition ' !instance ' is true.

Actually setting up the gridmap with a mesh library doesn't change anything.

Minimal reproduction project:
GridmapBug.zip

[akien-mga]

Cherry-picked for 3.0.7.