When using a GhostDefaultOverridesModifier I found that clicking the Update Components button in the Ghost Authoring Component will throw the following exception:
NullReferenceException: Object reference not set to an instance of an object
Unity.NetCode.Editor.GhostAuthoringComponentEditor.ExtractComponentInfo (UnityEngine.GameObject gameObject, Unity.NetCode.Editor.GhostAuthoringComponentEditor+SerializedComponentData& componentData) (at Packages/com.unity.netcode@0.6.0-preview.7/Editor/GhostAuthoringComponentEditor.cs:481)
This exception is only thrown when the GhostDefaultOverridesModifier does not have a GhostFieldModifier defined for one or more fields for a component.
Solution / Work around:
Defining a GhostFieldModifier for all fields in a component fixes the issue. If you have a component you do not want to sync, then set GhostFieldAttribute.SendData to false.