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.
render() {
        return <div />;
      }
      componentWillUnmount() {
        throw new Error('Hello');
      }
    }
    function Foo() {
      ops.push('Foo');
      return null;
    }
    ReactNoop.render(<BrokenComponentWillUnmount />);
    ReactNoop.flush();
    expect(() => {
      ReactNoop.render(<div />);
      ReactNoop.flush();
    }).toThrow('Hello');
    ops = [];
    ReactNoop.render(<Foo />);
    ReactNoop.flush();
    expect(ops).toEqual(['Foo']);
  });
  it('continues work on other roots despite caught errors', () => {
    class ErrorBoundary extends React.Component {
      state = {error: null};
      componentDidCatch(error) {