Error message: Unable to get property '_handlesCollection' of undefined or null reference
when RenderMode=Lightweight

Workarounds:
- enable the default behaviors to enable the Resize (and thus- maximize0 functionality, restore behaviors later:
				function OpenForm(sender, args) {
					var oWnd = radopen(null, "existing");
					var currBehaviors = oWnd.get_behaviors();
					oWnd.set_behaviors(Telerik.Web.UI.WindowBehaviors.Default);
					if (!oWnd.isMaximized()) {
						oWnd.maximize();
					}
					oWnd.set_behaviors(currBehaviors);
					
				}
- OR, have the Behaviors property contain either of the Maximize or Resize behavior. Or use the Default value

			<telerik:RadWindowManager runat="server" ID="rwm1">
				<Windows>
					<telerik:RadWindow runat="server" ID="existing" Behaviors="Close, Resize">
					</telerik:RadWindow>
				</Windows>
			</telerik:RadWindowManager>

			<telerik:RadButton ID="Button5" Text="open existing rw from rwm 3" AutoPostBack="false" OnClientClicked="OpenForm" runat="server"></telerik:RadButton>

			<script>
				function OpenForm(sender, args) {
					var oWnd = radopen(null, "existing");
					
					
				}
				function OnClientShow(sender, args) {
					if (!sender.isMaximized()) {
						sender.maximize();
					}
				}
			</script>