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.
firstUncaughtError = error;
    }
    return null;
  }
  function hasCapturedError(fiber: Fiber): boolean {
     
     
    return (
      capturedErrors !== null &&
      (capturedErrors.has(fiber) ||
        (fiber.alternate !== null && capturedErrors.has(fiber.alternate)))
    );
  }
  function isFailedBoundary(fiber: Fiber): boolean {
     
     
    return (
      failedBoundaries !== null &&
      (failedBoundaries.has(fiber) ||
        (fiber.alternate !== null && failedBoundaries.has(fiber.alternate)))
    );
  }
  function commitErrorHandling(effectfulFiber: Fiber) {
    let capturedError;
    if (capturedErrors !== null) {
      capturedError = capturedErrors.get(effectfulFiber);
      capturedErrors.delete(effectfulFiber);
      if (capturedError == null) {
        if (effectfulFiber.alternate !== null) {