Repro React 19.2.0 / Next latest

This is a minimal reproduction of an issue with React 19.2.0 and Next.js latest version.

Issue: reactjs/react.dev#8034 Potential fix: reactjs/react.dev#8037

---

Uses the following code

import { useState, Activity } from "react";

//...

  const [isShowingSidebar, setIsShowingSidebar] = useState(true);

  <Activity mode={isShowingSidebar ? "visible" : "hidden"}>
    <Sidebar />
  </Activity>

to produce the error:

Element type is invalid: expected a string (for built-in components) or a class/function (for composite components) but got: undefined.