Steps to reproduce:
1. Create a new package or use an existing one (possible to just create a new folder and add a package.json file to it containing example information from https://docs.unity3d.com/Manual/upm-manifestPkg.html)
2. Create a new project in the package folder
3. Create a new project outside of the package
4. Import the package containing the previously created project into the new project using Package Manager (press "+" at the top left -> Add package from disk...)
5. If it didn't crash after importing the package, delete the project folder from the package (outside of Unity)

Reproducible with: 2019.1.0a1, 2019.3.16f1, 2019.4.3f1, 2020.1.0b14, 2020.2.0a16
Not reproducible with: 2018.4.24f1

Notes:
- In 2018.4 after importing the package this warning is spammed:
"You are trying to import an asset which contains a global game manager. This is not allowed."
- Before the crash there's usually this assertion failure in the logs:
"Assertion failed on expression: 'gSingleton != NULL'"