I am not able to filter the subgrid inline lookup based on the customer  i tried by using the javascript,i.e, by using document.getElementById ,I am getting error below is my code please any body suggest me

function setSubgridLookupFiltering()
var subgridAddButtonId = "AssetRegister";

//Try to get the element from both the current and the parent document.
var subgridAddButton = document.getElementById(subgridAddButtonId) || window.parent.document.getElementById(subgridAddButtonId);

//This script may run before the subgrid has been fully loaded on the form. If this is the case,
//delay and retry until the subgrid has been loaded.
if (subgridAddButton == null) {
setTimeout(setSubgridLookupFiltering, 2000);

//Local function to retrieve the lookup control and apply the filter. We will queue this function in the click event handler of the
//Add button's click event.
var getSubgridLookupAndAddFilter = function() {
var subgridLookup = Xrm.Page.getControl("lookup_AssetRegister");

//Delay and retry until we can locate the lookup.
if (subgridLookup == null) {
setTimeout(getSubgridLookupAndAddFilter, 200);

//This is a custom property we are tagging on to the lookup control to ensure that we will
//apply the custom filter only once when the Add button is first clicked.
if (subgridLookup.customFilterAdded) {

subgridLookup.addPreSearch(function() {
//Standard logic to build up the filter query string
var filterQuery = "<fetch version='1.0' output-format='xml-platform' mapping='logical' distinct='false'>"+
"<entity name='new_assetregister'>"+
"<attribute name='new_name' />"+
"<attribute name='createdon' />"+
"<attribute name='new_assetregisterid' />"+
"<order attribute='new_name' descending='false' />"+
"<filter type='and'>"+
"<condition attribute='new_customer' operator='eq' uiname='Erbrains test' uitype='account' value='{3539838C-8675-E911-A956-000D3AF0694E}' />"+

//Standard call to add filter to the lookup control
subgridLookup.addCustomFilter(filterQuery, "new_assetregister");

//Mark the fact that we have already added the filter so that we won't do it again the next time the user clicks the Add button.
subgridLookup.customFilterAdded = true;

//Attach the function to retrieve the lookup and apply the filter to the Add button's click event. Remember that we
//can only get the lookup after the user has clicked the Add button.
subgridAddButton.addEventListener("click", function() {
setTimeout(getSubgridLookupAndAddFilter, 200);