Thank you mohammadreza khorasani! I have another errors but your solution helps me too.
My exception:
InvalidOperationException: Sequence contains no matching element System.Linq.Enumerable.Single[TSource] (System.Collections.Generic.IEnumerable1[T] source, System.Func2[T,TResult] predicate) (at <00000000000000000000000000000000>:0) Microsoft.AspNetCore.SignalR.Client.HubConnection..cctor () (at <00000000000000000000000000000000>:0)...