Change on the type used on casting of child element from
[mx.core.Container] to [mx.core.INavigatorContent], according to the
specification of the [mx.containers.ViewStack] component;
Also, change on the casting from (Type Checking) with [as] operator
to (Type Conversion) with [Class(arg)].
With the old class type [mx.core.Container] the casting by type
checking throw an [TypeError: Error #1009: null has no properties.],
instead of correct error that should be [TypeError: Error #1034: Type
Coercion failed: cannot convert %a to %t]