Fix error when loading a non-cached non-local asset

The error condition was triggered when the following
conditions were met:

- Load a non-local asset, i.e. selecting a filter list which
  is not part of the package

- The non-local asset is not in the local cache

dist/version

@@ -1 +1 @@
-1.19.2
+1.19.4

src/js/assets.js

@@ -693,6 +693,10 @@ api.get = function(assetKey, options, callback) {
     } else if ( typeof callback !== 'function' ) {
         callback = noopfunc;
     }
+    // This can happen if the method was called as a thenable.
+    if ( options instanceof Object === false ) {
+        options = {};
+    }
 
     return new Promise(resolve => {
     // start of executor