getSelectedIds always return null

I'm trying to get selevted Id's in an object and use the trigger in the documentation.

var arr = rbv_api.getSelectedIds("order");
var buff = '';
if (arr!=null) {
for (var k=0; k<arr.length; k++)
buff += arr[k]+', ';
}
return buff;


But ut will always return 'NULL'

v. 4.5.3.

Anyone who knows what the trick is?

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