尝试运行Microsoft示例程序ContactManager时出错

Question

当我尝试运行asp.net core 2.1的Contact Manager程序时，程序尝试为数据库设定种子时出现ArgumentNullException错误。

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)`.

repo是asp.net core 2.1文档的一部分，可以在这里找到：https://docs.microsoft.com/en-us/aspnet/core/security/authorization/secure-data?view=aspnetcore-2.1