how to solve NullReferenceException exception in Log.Error

Question

My C# code is like this

protected void Application_Error(object sender, EventArgs e)
{
var ex = Server.GetLastError();
Log.Error(ex);
var httpStatusCode = (ex is HttpException) ? (ex as HttpException).GetHttpCode() : 500;
var httpContext = ((MvcApplication)sender).Context;
httpContext.ClearError();
httpContext.Response.Clear();
httpContext.Response.StatusCode = httpStatusCode;
var shouldHandleException = true;
Exception exceptionToReplace;
}

And Log.Error(ex); throws this

 

System.NullReferenceException: Object reference not set to an instance of an object

error cause var ex = Server.GetLastError(); is null .

 

My question is how to solve this error?Any info will be helpful regarding this