error getValue in javascript CRM 365

Hi,

I'm try to retrieve data based on lookup in custom form by using javascript.

this is my code

// JavaScript source code
function GetActivityObjectiveType()
{
var lookupObj = new Array();
lookupObj = Xrm.Page.getAttribute("ibizcs_activityobjective").getValue();
//check for the value
if (lookupObj != null)
{
var Name = lookupObj[0].Name;
var ID = lookupObj[0].id;
var EntityType = lookupObj[0].entityType;
alert(Name);
}
}

when I run the form, and choose the field "ibizcs_activityobjective", it shows an error "Unable to get property 'getValue' of undefined or null".

how to solve this issue? I confirm that the field is on the form.