Non Editable Drop Down Field

Hi Team, I'm trying to make a drop down field to non-editable for all users except some Identity. Below was my code Event: On Form Populated //debugger; var identityList = aras.getIdentityList(); var field = getFieldByName("input_type"); alert(field); if (!identityList.includes("56A96DA9E981481688563E2D14D5D878")) //ID of the identity { field.getElementsByTagName("input")[0].disabled = true; } else { }   Error: Event handler failed with message: TypeError: Cannot set property 'disabled' of undefined [function onformpopulated$user$handler(e) { try { if (onformpopulated1(e) === false) return false; if (onformpopulated2(e) === false) return false; } catch (exp) { aras.AlertError(aras.getResource('', 'ui_methods_ex.event_handler_failed'), aras.getResource('', 'ui_methods_ex.event_handler_failure_msg', exp.description ? exp.description : (exp + ' [' +arguments.callee.toString()+ ']')), aras.getResource('','common.client_side_err')); return false; } }]