UI Toolkit Custom Inspector Disappeared in Playmode Unity - unity3d

Question

I have created a custom inspector using the UI Toolkit. The way I initialize the UI Document is as follows:
public VisualTreeAsset uXML;
This uXML is assigned through the inspector. Then in CreateInspectorGUI:
root = new VisualElement();
if (uXML == null)
{
Debug.Log("UI Document is " + documentName + " null");
return root;
}
uXML.CloneTree(root);
The root is the main root of the UI document (I am using UI Toolkit).
The problem is that when I enter play mode, the Inspectors disappears as shown:

Answer 1

I’m going to assume that you used a “Default Reference” for your editor script. Unfortunately, the default reference is only deserialised when you're in "Edit" mode. As soon as you go to "Play" mode, the reference is null.
The general way to get around the issue is to search for your asset when you create the inspector window:AssetDatabase.LoadAssetAtPath<VisualTreeAsset>("Assets/PathToYourFiles/YourFile.uxml")
Unfortunately, if you move your files somewhere else, you need to go in an update the string file location. Because it's a string path, there's no reference to follow the file if you move or rename the file. It's not clean, but it's easy, and gives you your inspector when in Play mode.
I did a quick search on the issue. Looking at this document, it seems Unity also suggest using Addressables or the Resources folder.
Because you don't really want the assets to follow you into a final build, I prefer placing them into an Editor folder, and just making sure I'm careful when moving or renaming the files. In one project, I made an automated process that "found" the file for me. It involved using a ScriptableObject and Assembly Reloads but I'm still in two minds whether the effort was worth it.