Reproduction steps:
0. Use one of the Unity versions marked as Reproducible with
1. Connect attached project "pre_checkout_callback_exception_test.zip" to Plastic SCM server
2. Open attached project and scene "SampleScene"
3. Observe Console window

Expected result: No exceptions
Actual result: "User-created pre-checkout callback has thrown an exception: Object reference not set <...>" and "Could not save package settings" errors

Reproducible with: 1.2 (2020.2.0a12, 2020.2.1f1, 2020.2.2f1, 2021.1.0b3, 2021.2.0a2)
Not reproducible with: 1.2 (2020.1.17f1, 2020.2.0a11)
Could not test with: 2018.4.29f1, 2019.4.14f1 (console errors break project)

Workaround found:
1. Comment out the following line in the file VCSExtension.cs
UnityEditor.VersionControl.Provider.preCheckoutCallback = preCheckoutCallback;
2. Go back to Unity. It will recompile the assets. Then the issue is not reproduced anymore.
3. Uncomment the previously commented line. Now all works as expected.

It seems that somehow the way of assigning the preCheckoutCallback changed. So when compiling with a previous Unity version (for example 2020.1.8f1), and then opening the project with a newer Unity version (for example 2020.2.1f1) reproduces the issue.

We should make a change to the Editor to protect the NullReferenceException in that scenario.