Adding null checks to StopReceiving methods of clients

TelegramBotBase/Base/MessageClient.cs

@@ -111,7 +111,7 @@ public class MessageClient
 
     public virtual void StopReceiving()
     {
-        _cancellationTokenSource.Cancel();
+        _cancellationTokenSource?.Cancel();
     }
 
 

TelegramBotBase/Base/ThreadPoolMessageClient.cs

@@ -81,7 +81,7 @@ public class ThreadPoolMessageClient : MessageClient
 
     public override void StopReceiving()
     {
-        _cancellationTokenSource.Cancel();
+        _cancellationTokenSource?.Cancel();
     }