Error when trying to run Microsoft sample program ContactManager

When I try to run the Contact Manager program for asp.net core 2.1, I get an ArgumentNullException error when the program tries to seed the database.

System.AggregateException
  HResult=0x80131500
  Message=One or more errors occurred.
  Source=ContactManager
  StackTrace:
   at ContactManager.Program.Main(String[] args) in C:\Users\EphraimF\source\repos\CMfinal\Program.cs:line 41

Inner Exception 1:
ArgumentNullException: Value cannot be null.

The error in generated because a null is returned by `userManager.FindByIdAsync(uid)`.

The repo is part of the asp.net core 2.1 documentation and can be found here: https://docs.microsoft.com/en-us/aspnet/core/security/authorization/secure-data?view=aspnetcore-2.1