How to prevent .length == 'undefined' error when populating dropdown via on-open

I'm dynamically loading the data into the dropdown upon a user opening a dropdown

I use on-open="loadDataIntoDropdown()" and it works great but it throws up the following error into the console:

`angular.js:12520 TypeError: Cannot read property 'length' of undefined
    at Scope.$scope.refreshButton (isteven-multi-select.js:521)
`
