DropdownList should support react StrictMode

[dimaqq]

I'm trying to use DropdownList in a new react project (newest react, hooks, etc.).

Code can be reduced to: <DropdownList data={["aa", "bb"]} defaultValue={null}/>

When I click the widget, react complains of:

index.js:1446 Warning: findDOMNode is deprecated in StrictMode. findDOMNode was passed an instance of Transition which is inside StrictMode. Instead, add a ref directly to the element you want to reference.

    in div (created by Transition)
    in Transition (created by SlideDownTransition)
    in SlideDownTransition (created by Popup)
    in Popup (created by DropdownList)
    in div (created by Widget)
    in Widget (created by DropdownList)
    in DropdownList (created by Uncontrolled(DropdownList))
    in Uncontrolled(DropdownList) (at UserCard.js:33)
    in [my component] (created by Context.Consumer)