You need to include a reference to your font awesome font library in the index.html. I usually just include the following:
<link rel="stylesheet" href="styles/css/custom/font-awesome.min.css">
That's if i am using it locally otherwise you can include.
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css">
The widget should really load the font libary, but i think this is an out of date widget that needs a bit of updating.
I hope this helps?
Regards
Simon