This bug was encountered using a popular Asset Store tool called Fungus. It's a sort of visual scripting tool for dialogues and quests.
It uses a custom made "flowchart" system.
When upgrading from 2019.3 to 2020.1,it seems like these Flowchart objects have lost all their references to script objects. It seems to occur as I encounter a package incompatibility error. The error can easily be fixed by Resetting Packages to Default.
But after the error gets fixed, the Flowchart loses all its data.

To see how it looks in the original version:
- download this small project in 2019.3
- Open the Conversation scene
- click on the Flowchart prefab
- Open Flowchart Window (if it's not already open)
- Notice the Start state has a <Game Started> Event
- If you press play, there will be a short "conversation" on the screen.

To reproduce this bug:
- download and upgrade the project to 2020.1.0b3 or 2020.2.0a4
- repeat the same steps to get to the Flowchart window
- notice the associated Event is gone
- pressing play you will get a lot o warning in the console and the conversation will not happen any more

Reproducible with: 2020.1.0a9, 2020.1.0b3 and 2020.2.0a4
Not reproducible with 2013.7f1 or earlier and 2020.1.0a8

This issue was found when upgrading a customer project - Midi the Cat, that uses Fungus.