Instance Scene Here doesn't work without node selected

[KoBeWi]

Godot version:

ed2f51b

Issue description:

When you Ctrl + Click inside 2D editor and use the Instance Scene Here option

it won't work when you don't have any node selected in scene tree. After selecting scene you will see an error that a parent node doesn't exist.

When you select the other option (Add Node Here), the node will be added to scene root if nothing is selected. Instancing should do the same.

For reference, this functionality was added in #41437