Question Manual-conforming custom MonoBehaviour inspector errors on entering Play mode?

Hi there,

Sorry if this turns out to have a simple solution, but after getting familiar with making custom inspectors with IMGUI, I am now trying out UIToolkit and following the tutorials in the manual - this page in particular:

https://docs.unity3d.com/Manual/UIE-HowTo-CreateCustomInspector.html

The guide first shows referencing a UXML file in the Car_Inspector.CreateInspectorGUI() method using the AssetDatabase.LoadAssetAtPath() method and a hardcoded filename. The guide then explains why this isn't ideal, and instead suggests declaring a public VisualTreeAsset variable within CarInspector, and setting a default reference to the UXML asset in the Inspector instead.

My issue is that after following this advice and setting up a default reference to my UXML file, I get an error in Play mode. If I select a GameObject containing the MonoBehaviour component that I am making the custom inspector for, the VisualTreeAsset variable is null. I have no error with a hardcoded path technique.

On the one hand, this seems reasonable, as I think I know and understand why default references don't carry from the Editor into Scenes. On the other hand, it seems then that the recommended way of making a custom inspector with UIToolkit doesn't work for the very common occurence of a custom MonoBehaviour inspector in Play mode?

My instinct is that surely something on my end has gone wrong and that the guide is fine, but I'm feeling baffled as to what's gone wrong and where to start?

 

Ahh, I am a bit behind and using 2021.3.5f1 in that project. I understand how UIToolkit might've changed since then. That said, the corresponding 2021 manual still suggests the same default reference technique.