[JS] CC2017 .topics.add

You are presuming an index exists by using doc.indexes[0]

Are you sure about that ?

function addTopic() {  
  
var doc = app.documents[0];  
      
  //Referencing the first index in the collection
  var myIndex = doc.indexes[0];
  //Trying to resolve the reference and if not found, creates a new index instance
  !myIndex.isValid && myIndex = doc.indexes.add();
    var new_topic = doc.indexes[0].topics.add (app.selection[0].contents);  
             
    new_topic.pageReferences.add (app.selection[0], PageReferenceType.currentPage);  
}  
addTopic();

Please always state the exact error message you get. "An error" is totally not helpful.

Apart from that, there are several clear errors I can already spot.

1. Your braces do not match (wrong on line 1).

2. "silence" is not a property I recognize.

3. Didn't look it up but I don't thing "Add" is a known function, because (probably) all standard functions start with a lowercase letter.