Three points from discussion in https://github.com/reactjs/react-redux/issues/756:

* Due to https://bugs.chromium.org/p/chromium/issues/detail?id=701371, environments like CodeSandbox show `Error: null` in our “React caught an error” warning. Let’s consider skipping both as they’re not very useful and look like a bug. `Error: null` looks like a separate error and draws attention away from the real error just above.

* In any case we should probably not print `The error was thrown at:` if we don’t have a JS stack.

* There is a related issue. What do we pass to error boundary if we caught `null`? It seems that most error boundaries will intuitively do something like `if (this.state.error)` or read `error.message`, which `null` will break. There are other falsy values too like `''` and `undefined`. My proposal is we always wrap primitives into our own `Error` objects that we pass to the boundary.
nextUnitOfWork = performFailedUnitOfWork(boundary);
    
    workLoop(minPriorityLevel, deadline);
  }
  function performWork(
    minPriorityLevel: PriorityLevel,
    deadline: Deadline | null,
  ) {
    if (__DEV__) {
      startWorkLoopTimer();
    }
    invariant(
      !isPerformingWork,
      'performWork was called recursively. This error is likely caused ' +
        'by a bug in React. Please file an issue.',
    );
    isPerformingWork = true;
    nestedUpdateCount = 0;
    
    
    const previousPriorityContext = priorityContext;
    let didError = false;
    let error = null;
    if (__DEV__) {
      invokeGuardedCallback(null, workLoop, null, minPriorityLevel, deadline);
      if (hasCaughtError()) {
        didError = true;
        error = clearCaughtError();
      }