JI-1285 Add comment

js/h5p-x-api-event.js

@@ -142,7 +142,13 @@ H5P.XAPIEvent.prototype.setObject = function (instance) {
     }
   }
   else {
-    // Empty object
+    // Content types view always expect to have a contentId when they are displayed.
+    // This is not the case if they are displayed in the editor as part of a preview.
+    // The fix is to set an empty object with definition for the xAPI event, so all
+    // the content types that rely on this does not have to handle it. This means
+    // that content types that are being previewed will send xAPI completed events,
+    // but since there are no scripts that catch these events in the editor,
+    // this is not a problem.
     this.data.statement.object = {
       definition: {}
     };