Help Wanted: Unity UIDocument Using Wrong Input System

I'm working on a game in which I'm attempting to use the new UI Toolkit (1.0.0-preview.18) and the "new" Input System. When I try and run the game I'm getting the following exception:

"InvalidOperationException: You are trying to read Input using the UnityEngine.Input class, but you have switched active Input handling to Input System package in Player Settings.​"

I have created an EventSystem object which uses the InputSystemUIInputModule as its script, as specified in the docs / forums (e.g. https://docs.unity3d.com/Packages/com.unity.inputsystem@1.0/manual/UISupport.html). However my UI Documents are for some reason still using the default event system instead of the one I've created.

Is there some further step I need to do to complete to register the Event System object with the UI Toolkit documents? Any help would be greatly appreciated.

Event System in project: