title: Only one instance of a scriptmanager can exist on a page
body: <p>I design an ASP.NET web usercontrol and with a maskeditor and scriptmanager, I always get an object reference not set to an instance of an object exception at runtime.</p>
  
  <p>Stacktrace is:</p>
  
  <p>[InvalidOperationException: Only one instance of a ScriptManager can be added to the page.]
     System.Web.UI.ScriptManager.OnInit(EventArgs e) +384613
     System.Web.UI.Control.InitRecursive(Control namingContainer) +333
     System.Web.UI.Control.InitRecursive(Control namingContainer) +210
     System.Web.UI.Control.InitRecursive(Control namingContainer) +210
     System.Web.UI.Control.InitRecursive(Control namingContainer) +210
     System.Web.UI.Control.InitRecursive(Control namingContainer) +210
     System.Web.UI.Control.InitRecursive(Control namingContainer) +210
     System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +378</p>
  
  <p>What causes this?
  Thanks</p>