Adding the [UxmlObjectReference] attribute to a field that has a type that is not UxmlObject causes errors. We should improve this error message to explain that it is an incompatible type.

E.G

{code:java}
//[UxmlObject] // uncomment to fix
public class MyObject
{
    [UxmlAttribute]
    public float myFloat;
}

[UxmlElement]
public partial class MyElement : VisualElement
{
    [UxmlObjectReference("my-object")]
    public MyObject myObj;
}{code}