How to reproduce:
1. Open the attached project "1206352.zip"
2. Open the SampleScene
3. Select the Container script (Assets -> Runtime -> Container)
4. Reimport the script (right-click -> Reimport)
5. Enter Play mode and notice Default value being logged
6. Exit Play mode and uncomment the [SerializeReference] attribute of the Container script
7. Enter Play mode

Expected result: Default value is logged
Actual result: NullReferenceException is thrown

Reproducible with: 2019.3.0f4, 2020.1.0a18

Notes:
Reimporting the script when [SerializeReference] attribute is uncommented will stop the Item class constructor from being invoked.
Creating a new ScriptableObject (right-click -> Create -> Container) will Invoke the constructor.
[SerializeReference] attribute was not available prior to Unity 2019.3, therefore the issue is not reproducible.