we should specify URL, otherwise it will throw NRE

src/lib/PnP.Framework/AuthenticationManager.cs

@@ -899,7 +899,7 @@ public async Task<string> GetAccessTokenAsync(string[] scopes, CancellationToken
                         {
                             throw new ArgumentException($"{nameof(GetAccessTokenAsync)}() called without an ACS token generator. Specify in {nameof(AuthenticationManager)} constructor the authentication parameters");
                         }
-                        return acsTokenGenerator.GetToken(null);
+                        return acsTokenGenerator.GetToken(uri);
                     }
                 case ClientContextType.AccessToken:
                     {