Visual Scripting: "Invalid returns count" error on CallInstance

[netcell]

Operating system or device, Godot version, GPU Model and driver (if graphics related):
Windows 10, Godot 3 built on commit 454dec2

Issue description:
Call node on Instance mode throws this error on some functions:

Invalid returns count for call_mode == CALL_MODE_INSTANCE

This doesn't happen with Node mode on Call node

Steps to reproduce:
Drag in a reference of Node
Instance a reference of another Node
Drag in a Call node
Set Call Mode to Instance
Set Base Type to Node
Set Function to add_child
Plug the first Node into instance port
Plug the second Node into node port
Run

[reduz]

@bojidar-bg want to give it a try?