[lvgl] Fix msgbox content (#7912)

tests/components/lvgl/lvgl-package.yaml

@@ -109,6 +109,10 @@ lvgl:
       close_button: true
       title: Messagebox
       bg_color: 0xffff
+      widgets:
+        - label:
+            text: Hello Msgbox
+            id: msgbox_label
       body:
         text: This is a sample messagebox
         bg_color: 0x808080
@@ -137,6 +141,9 @@ lvgl:
         - lvgl.widget.focus: mark
         - lvgl.widget.redraw: hello_label
         - lvgl.widget.redraw:
+        - lvgl.label.update:
+            id: msgbox_label
+            text: Unloaded
       on_all_events:
         logger.log:
           format: "Event %s"