Fix a bug in the Illustrator script

It got mad if a layer was missing.

scripts/illustrator.jsx

@@ -49,6 +49,10 @@ for (var i=0; i<settings.length; i++) {
         }
     }
 
+    if (layer === null) {
+        continue;
+    }
+
     var raster = layer.pageItems[0]
     var pluginRef = raster.trace()
     var trace = pluginRef.tracing