To repro:

• Start the app with npm start
• Click anywhere outside the modal to try to close it
• You'll see an error

The problem is that renderSubtreeIntoContainer is returning null instance of an instance. And this only happens on the first render, I think, so we only hit this problem if we conditionally render the Modal like this.state.open && <Modal> instead of <Modal open={this.state.open} />.