Georgi,
I am having the same issue. Copying your file and running it immeadiately caused the error with no other modification.
This is in the new q2 build
Sys.Application.add_init(function() {
$create(Telerik.Web.UI.RadWindow, {"_dockMode":true,"_stylezindex":"99999","behaviors":5,"clientStateFieldID":"ctl00_dgError_ClientState","formID":"aspnetForm","height":"300px","iconUrl":"","minimizeIconUrl":"","name":"dgError","skin":"Windows7","width":"500px"}, null, null, $get("ctl00_dgError"));
});
This is in the previous q1 build
Sys.Application.add_init(function() {
$create(Telerik.Web.UI.RadWindow, {"_dockMode":true,"_stylezindex":"99999","behaviors":5,"clientStateFieldID":"ctl00_dgError_ClientState","formID":"aspnetForm","height":"300px","iconUrl":"","minimizeIconUrl":"","name":"dgError","skin":"Windows7","width":"500px"}, null, null, $get("ctl00_dgError"));
They are exactly the same, so something going on internally?