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>