ui/lvgl: fix a bug where a top container didn't have its object created

a previous commit 7d1ab5cb rewrote toplayer objects creation as
Container.newTop except the new code was using the top layer object
obtained from lv_disp_get_layer_top as is. this obviously led to all
sorts of UB and illegal memory access when screen.sleep'ing.

src/ui/lvgl.zig

@@ -482,7 +482,8 @@ pub const Container = struct {
     /// suitable for widgets like a popup window.
     pub fn newTop() !Container {
         const toplayer = lv_disp_get_layer_top(null);
-        return .{ .lvobj = toplayer };
+        const o = lv_obj_create(toplayer) orelse return error.OutOfMemory;
+        return .{ .lvobj = o };
     }
 
     /// applies flex layout to the container.