title: ASP.Net UpdatePanel ImageButton causes "this._postbackSettings.async is null or not an object"
body: <p>I get this error on an update panel within a popupControlExtender which is within a dragPanelExtender.</p>
  
  <p>I see that a lot of other people have this issue and have various fixes none of which have worked for me.</p>
  
  <p>I would love to hear a logical explanation for why this is occurring and a foolproof way to avoid such issues in the future.</p>
  
  <p>I have found that like others maintain this error does not occur when the trigger is a LinkButton rather than an ImageButton, still wondering if anyone has an explanation.</p>