React 17 gives findDomNode deprecated warnings

[theogravity]

Repro code:

    <SplitPane split="vertical" collapse={true}>
      <div>a</div>
      <div>b</div>
    </SplitPane>

Console gives:

Warning: findDOMNode is deprecated in StrictMode. findDOMNode was passed an instance of Transition2 which is inside StrictMode. Instead, add a ref directly to the element you want to reference. Learn more about using refs safely here: https://reactjs.org/link/strict-mode-find-node

[theogravity]

Duplicate of #96