handle early LoadURL messages without panicking by sending initial ur…

…l load

fixes #6126

components/compositing/compositor.rs

@@ -882,15 +882,13 @@ impl<Window: WindowMethods> IOCompositor<Window> {
     fn on_load_url_window_event(&mut self, url_string: String) {
         debug!("osmain: loading URL `{}`", url_string);
         self.got_load_complete_message = false;
-        let root_pipeline_id = match self.scene.root {
-            Some(ref layer) => layer.get_pipeline_id(),
-            None => panic!("Compositor: Received WindowEvent::LoadUrl without initialized compositor \
-                           layers"),
-        };
         let url = Url::parse(&url_string).unwrap();
         self.window.set_page_url(url.clone());
+        let msg = match self.scene.root {
+            Some(ref layer) => ConstellationMsg::LoadUrl(layer.get_pipeline_id(), LoadData::new(url)),
+            None => ConstellationMsg::InitLoadUrl(url)
+        };
 
-        let msg = ConstellationMsg::LoadUrl(root_pipeline_id, LoadData::new(url));
         let ConstellationChan(ref chan) = self.constellation_chan;
         chan.send(msg).unwrap()
     }