protected async Task LoginTest()
{
   await _SignInManager.SignInAsync(new ApplicationUser()
   { UserName = "test@test.com" }, true);
   UriHelper.NavigateTo("/", true);
}

System.InvalidOperationException: The response headers cannot be modified because the response has already started.
at Microsoft.AspNetCore.HttpSys.Internal.HeaderCollection.ThrowIfReadOnly()
at Microsoft.AspNetCore.HttpSys.Internal.HeaderCollection.set_Item(String key, StringValues value)
at Microsoft.AspNetCore.Http.Internal.ResponseCookies.Append(String key, String value, CookieOptions options)
at Microsoft.AspNetCore.Authentication.Cookies.ChunkingCookieManager.AppendResponseCookie(HttpContext context, String key, String value, CookieOptions options)
at Microsoft.AspNetCore.Authentication.Cookies.CookieAuthenticationHandler.HandleSignInAsync(ClaimsPrincipal user, AuthenticationProperties properties)
at Microsoft.AspNetCore.Authentication.AuthenticationService.SignInAsync(HttpContext context, String scheme, ClaimsPrincipal principal, AuthenticationProperties properties)
at Microsoft.AspNetCore.Identity.SignInManager`1.SignInWithClaimsAsync(TUser user, AuthenticationProperties authenticationProperties, IEnumerable`1 additionalClaims)
at WebApplication3.Pages.Account.Login.RegUser() in C:UsersdavidsourcereposWebApplication3WebApplication3PagesAccountLogin.razor:line 28
at Microsoft.AspNetCore.Components.ComponentBase.CallStateHasChangedOnAsyncCompletion(Task task)
at Microsoft.AspNetCore.Components.Rendering.Renderer.GetErrorHandledTask(Task taskToHandle)