OnActivate handler to:

1) Process delayed error messages from background processing
2) Set keyboard focus to the 3D scene if no wx window has keyboard focus.

src/slic3r/GUI/Plater.hpp

@@ -151,6 +151,8 @@ public:
 
     void on_extruders_change(int extruders_count);
     void on_config_change(const DynamicPrintConfig &config);
+    // On activating the parent window.
+    void on_activate();
 
     void update_object_menu();