This is working as designed. The developer defines the makeItem() callback function that returns a VisualElement to host the list view items as they are added to the list. The developer also defines a bindItem() callback function that binds the data from its source to the item that was created. But the problem here is that the makeItem() function also declares that it wants to be notified of value changes via RegisterValueChangedCallback() and when called, uses the objectField's userData property directly and casts to an integer. While normally this would work, the first time it is called (at the time of the exception in this example), the userData property is not yet initialized, so trying to cast null to an integer will fail. If the developer really wants to call this without initializing userData first, they can use the following code in their bindItem callback instead:

Action<VisualElement, int> bindItem = (e, i) =>
{
// instead of using ".value = ...", use ".SetValueWithoutNotify(...)"
(e as ObjectField).SetValueWithoutNotify(cellInfo.roomInfo.overrideRoomPrefabs[i]);
e.userData = i;
};

This will ensure that the assignment when the item is bound will not send a notification (so the registered value changed callback in makeItem won't be called).