FAQ: wrong Ref usage, missing expect() call (#76)

* FAQ: Added unwraping after `cast` call, so it compiles correctly.
* Adjusted Rust class to `Option<Instance<T>>` so it fits with tutorial and compiles properly

Author: tuto193

src/faq/code.md

@@ -392,7 +392,7 @@ struct MyNode {
     // late-initialization is modeled with Option
     // the Default derive will initialize both to None
     node2d: Option<Ref<Node>>,
-    instance: Option<Ref<MyClass>>,
+    instance: Option<Instance<MyClass>>,
 }
 
 #[methods]
@@ -404,7 +404,8 @@ impl MyNode {
             .get_node("Node2D")
             .expect("this node must have a child with the path `Node2D`");
         let node2d = unsafe { node2d.assume_safe() };
-        let node2d = node2d.cast::<Node2D>();
+        let node2d = node2d.cast::<Node2D>()
+                        .expect("child must be of type 'Node2D'");
         self.node2d = Some(node2d.claim());
 
         // Get an existing child node that is a Rust class.