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.
'flakiness. React does its best to preserve the "Pause on ' +
              'exceptions" behavior of the DevTools, which requires some ' +
              "DEV-mode only tricks. It's possible that these don't work in " +
              'your browser. Try triggering the error in production mode, ' +
              'or switching to a modern browser. If you suspect that this is ' +
              'actually an issue with React, please file an issue.',
          );
        }
        ReactErrorUtils._hasCaughtError = true;
        ReactErrorUtils._caughtError = error;
      } else {
        ReactErrorUtils._hasCaughtError = false;
        ReactErrorUtils._caughtError = null;
      }
      
      window.removeEventListener('error', onError);
    };
    invokeGuardedCallback = invokeGuardedCallbackDev;
  }
}
let rethrowCaughtError = function() {
  if (ReactErrorUtils._hasRethrowError) {
    const error = ReactErrorUtils._rethrowError;
    ReactErrorUtils._rethrowError = null;
    ReactErrorUtils._hasRethrowError = false;
    throw error;
  }
};
module.exports = ReactErrorUtils;