Having copied the example snippet for the new [SerializeReference] feature from:
Creating a child<>parent relationship through the [SerializeReferenceAttribute] causes the Unity Editor to freeze, when trying to expand it in the Inspector.
Reproduce
Open attached project
Right-click in project window and choose “Create > Some Thing”
Select “New Some Thing” in project window
Click from main menu “BugReport/Populate Selected Some Thing”
Hold down the ALT key and click the foldout icon to expand “Trees” in the Inspector