The CommandArgument value of RadButton is not available in the ClientClicking eventargs of RadButton. The get_commandName() method of the event arguments always returns null.
The issue is introduced in Q1 2016 release.
Steps to reproduce:
Run the following code and click the button.
    <script type="text/javascript">
        function btnEdit_Clicking(sender, eventArgs) {
            alert(eventArgs.get_commandArgument());
        }
    </script>
    <telerik:RadButton ID="RadButton2" AutoPostBack="false"
        runat="server" Text="Click Me"
        OnClientClicking="btnEdit_Clicking"
        CommandArgument="Name">
    </telerik:RadButton>