In linkify function you must add this:

if (!_str) return "";

before the _str.replace to avoid unexpected errors.

Also, i'm trying to modify the function to add hg-click attribute instead of using the href but i can't understand why it's removed by angular. The function returns the element right, with the ng-click attribute but then it's not rendered on the page. I'm using ng-bind-html="item.text | linkify"