in app/Data/AppUserStore.cs [132:140]
public Task SetUserNameAsync(AppUser user, string userName, CancellationToken cancellationToken)
{
cancellationToken.ThrowIfCancellationRequested();
if (user == null) throw new ArgumentNullException(nameof(user));
if (userName == null) throw new ArgumentNullException(nameof(userName));
user.UserName = userName;
return Task.FromResult<object>(null);
}