Describe the bug
Whilst upgrading Boss Room to MLAPI develop, we encountered an issue with the Network Manager and its NetworkPrefabs container, where if we delete any of the prefabs or variants that were added to the NetworkPrefabs list, then save (apply changes to) the NetworkingManager prefab, then shut down Unity and load it up again, the NetworkPrefabs list will be invalidated, most of the elements will have a null value., see screenshot below!

To Reproduce
Steps to reproduce the behavior:

1. Open 'Startup' scene in Boss Room
2. Click on 'NetworkingManager'
3. Remove an element from the NetworkPrefabs list
4. Save/Apply changes to prefab + save the scene.
5. Close and Relaunch Unity

Actual outcome
The NetworkPrefabs container's elements are invalidated, set to null after executing the steps above and relaunching the editor!

After adding one test prefab

After removing any prefab

Expected outcome
We should be able to freely remove/add any of the Prefabs/Variants to the Network Prefabs container in any order

Environment (please complete the following information):

• OS: [MacOS]
• Unity Version: [2020.3]
• MLAPI Version: [develop]
• MLAPI Commit: [https://github.com/Unity-Technologies/com.unity.multiplayer.mlapi/commit/fa2109e7e54cb15c96661a03506d6bbb7bcda517]