removePreSearch Lookup

Hi All, 

I'm trying to add PreSearch on lookup field incase of an option in optionset field and trying to remove it on another option. Add is working fine but removePreSearch isn't

onChangeProductType: function (executionContext) {

var formContext = executionContext.getFormContext();

if (formContext.getAttribute("ab_producttype").getValue() == 108730002) {
formContext.getControl("ab_product").addPreSearch(function () {
filterlookup(executionContext);
});
}
else {

formContext.getControl("ab_product").removePreSearch(filterlookup);

}
},

filterlookup: function (executionContext) {

var formContext = executionContext.getFormContext();

var filter = "<filter type='and'><condition attribute = 'name' operator = 'like' value = '%Builders%' /><condition attribute='name' operator='like' value='%Single%' /><condition attribute='name' operator='like' value='%60%' /><condition attribute='name' operator='like' value='%Exterior%' /></filter>";

formContext.getControl("ab_product").addCustomFilter(filter);

},