Use checked handles for app dialogs

src/node_about.h

@@ -4,8 +4,8 @@
 
 class NodeAbout : public Node
 {
-    NodeButton* btn_ok;
 public:
+    NodeButton* btn_ok;
     virtual Node* clone_instantiate() const override;
     virtual void init() override;
     virtual kEventResult handle_event(Event* e) override;