fix: исправил ошибку, когда по email не удалось найти пользователя.

PhoneStore/Services/AccountService.cs

@@ -63,6 +63,8 @@ namespace PhoneStore.Services
         public async Task<IdentityResult> LogIn(LoginViewModel model)
         {
             User user = await _userManager.FindByEmailAsync(model.Email);
+            if (user is not null)
+            {
                 SignInResult result = await _signInManager.PasswordSignInAsync(
                     user,
                     model.Password,
@@ -71,7 +73,7 @@ namespace PhoneStore.Services
                 );
                 if (result.Succeeded)
                     return new IdentityResult {StatusCodes = StatusCodes.Success};
-            
+            }
             return new IdentityResult
             {
                 StatusCodes = StatusCodes.Error,