Found the problem by myself.
I set ClientIDMode="Static" at
<MasterTableView EnableHierarchyExpandAll="true" AllowMultiColumnSorting="True" ClientIDMode="Static" HierarchyLoadMode="Conditional">
and at
<telerik:GridTableView HierarchyLoadMode="Conditional" EnableHierarchyExpandAll="true" Width="100%" runat="server" ClientIDMode="Static" ShowHeader="False">
But this elements have no setter for ID. It generates onClick javascript where
$find("")
returns null.