Element "ion-side-menus" not allowed as child of element "body" in this context. (Suppressing further errors from this subtree.)

I am new to Ionic. I followed one of tutorial online.
in index.html,

...

In NetBeans, under tag causes error, "Element “ion-side-menus” not allowed as child of element “body” in this context. (Suppressing further errors from this subtree.)"
I searched online for a couple of days, but I still have no solution.
If anyone knows it, please shed some light on it.
thank you very much in advance,
Sara

I’m also facing the same issue in NetBeans.