The error message is pretty explicit:
Unable to get value of the property 'get_isLoading': object is null or undefined
Which means that from this:
var stillonLoadState = viewerReference.get_isLoading();
viewerReference is null.
It's loaded here:
var viewerReference = $find("ReportViewer1");
So $find is returning null.
Since it returns null is it can't find a control with that ID:
Matt Berseth: The Ever-Useful $get and $find ASP.NET AJAX Shortcut Functions
You need to look at your page and see what your report control is called...