How can i get message from telegram channel into my bot? - asp.net-web-api

Question

I'm using Telegeram.bot for develop simple telegram bot on .net
I added my bot(administrator) to public channel and i want to get message from channel ans resend again to bot users
but I have problem, here is my code
#region Properties
private Api bot;
string botToken = "Bot Token";
#endregion
#region Constructor
/// <summary>
/// Basic constructor for the api controller
/// </summary>
public WebhookController()
{
bot = new Api(botToken);
}
#endregion
public async Task<IHttpActionResult> Post(Update update)
{
var text = update.Message.Text;
var chatType = update.Message.Chat.Type;
if (chatType == ChatType.Channel)
{
// Send Text message to users that previusly we gathered their telegram id and saved them in our DB
// ******Problem is here******
}
else if (chatType == ChatType.Private)
{
if (text != null && text.Contains("/start") || text.Contains("start"))
{
await bot.SendTextMessage(chatId: update.Message.Chat.Id, text: "START");
}
}
else
{
await bot.SendTextMessage(chatId: update.Message.Chat.Id, text: "ERROR!!!");
}
return Ok();
}
public string Get()
{
return "Yes Its Work";
}
But I got 500 Internal Server Error
"Message": "An error has occurred.",
"ExceptionMessage": "Object reference not set to an instance of an object.",
"ExceptionType": "System.NullReferenceException",
"StackTrace": " at CodeBlock.Bot.Engine.Controllers.webhookController.<Post>d__11.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at System.Threading.Tasks.TaskHelpersExtensions.<CastToObject>d__3`1.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__0.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__2.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at System.Web.Http.Controllers.AuthenticationFilterResult.<ExecuteAsync>d__0.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n at System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__1.MoveNext()"