Drawing not allowed errors for valid draw calls on startup

[letheed]

When opening the editor the following error appears multiple times in the output console:

Drawing is only allowed inside NOTIFICATION_DRAW, _draw() function or 'draw' signal.

This happens when I make draw calls in a tool script attached to a node that is the child of a TileMap. Not sure why Tilemap, a simple Node2D doesn't trigger it but I haven't tried other nodes. The scene needs to be opened by the editor on startup as well. If I open it manually, there is no error.

I think this could be the same issue as #17625.

As I posted there, it seems drawing is false for a short while at the beginning of the script's run.

Godot version:
3.0.3

Minimal reproduction project:
Playground.zip