CoapServer Crashes after some works

[mrhb]

I received such an error. how can I discover it to know the origin of the error

An exception of type 'System.NullReferenceException' occurred in CoAP.NET.dll but was not handled in user code

 at CoAP.Stack.ReliabilityLayer.TransmissionContext.Start() in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\ReliabilityLayer.cs:line 289
  at CoAP.Stack.ReliabilityLayer.PrepareRetransmission(Exchange exchange, Message msg, Action`1 retransmit) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\ReliabilityLayer.cs:line 245
  at CoAP.Stack.ReliabilityLayer.SendResponse(INextLayer nextLayer, Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\ReliabilityLayer.cs:line 98
  at CoAP.Stack.LayerStack.NextLayer.SendResponse(Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\LayerStack.cs:line 168
  at CoAP.Stack.AbstractLayer.SendResponse(INextLayer nextLayer, Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\AbstractLayer.cs:line 40
  at CoAP.Stack.TokenLayer.SendResponse(INextLayer nextLayer, Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\TokenLayer.cs:line 49
  at CoAP.Stack.LayerStack.NextLayer.SendResponse(Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\LayerStack.cs:line 168
  at CoAP.Stack.AbstractLayer.SendResponse(INextLayer nextLayer, Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\AbstractLayer.cs:line 40
  at CoAP.Stack.BlockwiseLayer.SendResponse(INextLayer nextLayer, Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\BlockwiseLayer.cs:line 259
  at CoAP.Stack.LayerStack.NextLayer.SendResponse(Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\LayerStack.cs:line 168
  at CoAP.Stack.AbstractLayer.SendResponse(INextLayer nextLayer, Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\AbstractLayer.cs:line 40
  at CoAP.Stack.ObserveLayer.SendResponse(INextLayer nextLayer, Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\ObserveLayer.cs:line 121
  at CoAP.Stack.LayerStack.NextLayer.SendResponse(Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\LayerStack.cs:line 168
  at CoAP.Stack.AbstractLayer.SendResponse(INextLayer nextLayer, Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\AbstractLayer.cs:line 40
  at CoAP.Stack.LayerStack.StackTopLayer.SendResponse(INextLayer nextLayer, Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\LayerStack.cs:line 107
  at CoAP.Stack.LayerStack.SendResponse(Exchange exchange, Response response) in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Stack\LayerStack.cs:line 47
  at CoAP.Net.CoAPEndPoint.<>c__DisplayClass4.<SendResponse>b__3() in D:\Code\SmeshLink\CoAP.NET\CoAP.NET\Net\CoAPEndPoint.cs:line 219
  at System.Threading.Tasks.Task.InnerInvoke()
  at System.Threading.Tasks.Task.Execute()