How to send response messages from a server using web socket in C#?

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C#

ASP.NET

WebSocket-server

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I want to send response messages from a server using web socket in c#. But am getting the exception as below:
Object reference issue.

From client to server i am able to send message using client web socket. But from server I am unable to send the message from client.

But  am getting the exception as below: 
Object reference issue. 

What I have tried:

Core Code of Client and Server sending messages
Client Level:

if (webSocket.State == WebSocketState.Open)
                  {
                  //Console.WriteLine("Write some to send over to server..");
                  //string stringtoSend = Console.ReadLine();
                  string stringtoSend = txtOutgoing.Text;

                  byte[] buffer = encoding.GetBytes(stringtoSend);

                  await webSocket.SendAsync(new ArraySegment<byte>(buffer), WebSocketMessageType.Binary, false, CancellationToken.None);
                  // Console.WriteLine("Sent:     " + stringtoSend);


                  await Task.Delay(1000);
              }

Server level:

WebSocketContext webSocketContext = null;
            try
            {
                // while (webSocket.State == WebSocketState.Open)
                WebSocket webSocket = webSocketContext.WebSocket;

                //Console.WriteLine("Write some to send over to server..");
                //string stringtoSend = Console.ReadLine();
                string stringtoSend = txtOutgoing.Text;

                byte[] buffer = encoding.GetBytes(stringtoSend);

                await webSocket.SendAsync(new ArraySegment<byte>(buffer), WebSocketMessageType.Binary, false, CancellationToken.None);

            }
            catch (Exception ex)
            {

            }