As Darell said, most probably your treeview client (javascript) ID is different from the server-side "trvRelationinfra" ID. This is controlled by the framework (INamingContainer) and we cannot change that.

There a couple of approaches:

1. You can use directly the ClientID of the treeview using the following syntax:

var tree = igtree_getTreeById("<%= trvRelationinfra.ClientID %> "); 

or

2.  Use the client-side InitializeTree event to get an instance of the tree based on the client id and then use from this point thereafter:

<ignav:UltraWebTree ID="UltraWebTree1" runat="server" Editable="true"> 
        <ClientSideEvents InitializeTree="initTree" />            
         ...
    </ignav:UltraWebTree>
   
    <script language="javascript">
   
    var tree;
   
    function initTree(treeId)
    {      
        tree = igtree_getTreeById(treeId);
    }
   
    </script>