Changing history in loaded graph doesn't always work.

[yochrisbolton]

Godot version:
Godot 3.1 beta 4

OS/device including version:
Ubuntu 18.04.1 LTS x86_64

Issue description:
When changing history ( undo / redo ) in a loaded graph, it can crash the program. It seems like we're not logging the information correctly.

Steps to reproduce:
Open a graph, move some Dialogue nodes around, hit undo a few times and the program will crash.