Make cx.spawn() return an Option<TaskId>, returns None if finished immediately

examples/compose.rs

@@ -19,7 +19,8 @@ fn app(cx: Scope) -> Element {
                 emails_sent.write().push(message);
             }
         }
-    });
+    })
+    .unwrap();
 
     cx.render(rsx! {
         div {
@@ -27,13 +28,7 @@ fn app(cx: Scope) -> Element {
 
             button {
                 onclick: move |_| {
-                    let dom = VirtualDom::new_with_props(compose, ComposeProps {
-                        app_tx: tx.clone()
-                    });
-
-                    // this returns a weak reference to the other window
-                    // Be careful not to keep a strong reference to the other window or it will never be dropped
-                    // and the window will never close.
+                    let dom = VirtualDom::new_with_props(compose, ComposeProps { app_tx: tx.clone() });
                     window.new_window(dom, Default::default());
                 },
                 "Click to compose a new email"