Shader Graph - Exception occurs when undoing deletion of a Gradient Node

If you delete a Gradient Node from a Graph and then undo, a UnityException will occur.

UnityException: set_mode is not allowed to be called during serialization, call it from OnEnable instead. Called from ScriptableObject 'GraphObject'.

1. Launch Unity
2. Open the attached project
3. Open the PBRGraph Shader Graph Asset
4. Add a Gradient Node to the Graph
5. Delet the Gradient Node
6. Undo

Expected Result:
The Gradient Node should be readded to the Graph and no errors should appear in the console.

Actual Result:
An exception appears in the console.

Tested on

2019.1.0a9 with Shader Graph version 5.0.0-preview and LWRP version 5.0.0-preview
2018.3.0ab10 with Shader Graph version 4.1.0-preview and LWRP version 4.1.0-preview

Occurs on Windows and MacOS.