Object reference not set to an instance of an object

i have created a windows app in c# it loads a web page from a site that contains image. there is a button at the side of the app. when i click on the button it calls a method which get the div tag of in which the image exists and save it as bmp on local drive. but when i click on the button it gives the following error.
 
Object reference not set to an instance of an object.
                  if (web != null)
                  {
 
                    
 
                  HtmlElement elems = web.Document.GetElementById("recaptcha_image")
                  }
please tell me what i need to do.