objectReference is null in prefab

Hello!

I've encountered an issue with my custom editor for my MonoBehaviour class. It seems I cannot instantiate ScriptableObjects (myQuestToEdit) and then assign them to a SerializedProperty (element) during prefab-editing. I've breakpointed the code and it seems like

element.objectReferenceValue

remains null after I assign it with

element.objectReferenceValue = ScriptableObject.CreateInstance();

This code does however work when I edit the object via the scene.

Here's a small sample of where it goes wrong. Any ideas?

 myQuestToEdit = ScriptableObject.CreateInstance<MoveToInteractableQuest>();
 questList.InsertArrayElementAtIndex(questList.arraySize);
 SerializedProperty element = questList.GetArrayElementAtIndex(questList.arraySize-1);
 element.objectReferenceValue = ScriptableObject.Instantiate(myQuestToEdit);