How to reproduce:
1. Create a new project
2. Create and open a new prefab
3. Start creating and attaching a new C# script while in prefab mode
4. Right after creating the script immediately leave prefab mode
5. Observe the console

Expected result: The prefab is saved correctly with the newly created script
Actual result: The prefab is not saved correctly and throws an error

Reproduced in: 2019.1.0a3, 2018.3.0b4

Note: Only reproduces in a short amount of time while the scripts are being compiled