ReactCompositeComponent.js

// When getting error: "Cannot read property 'componentWillUnmount' of null"
// It's because React is swallowing the exception without rethrowing it
// In ReactCompositeComponent.js, edit lines that look like this, and add the Catch part

if (canInstantiate) {
  if (process.env.NODE_ENV !== 'production') {
    ReactCurrentOwner.current = this;
    try {
      inst = new Component(publicProps, publicContext, ReactUpdateQueue);
    } catch (ex) {
      console.log(ex);
      debugger;
    } finally {
      ReactCurrentOwner.current = null;
    }
  } else {
    inst = new Component(publicProps, publicContext, ReactUpdateQueue);
  }
}