[fixed] Call parent.removeChild only if parent exists

[husseyexplores]

Fixes #769

Should we also check before calling removeChild here (In componentDidUpdate()?

const { prevParent, nextParent } = snapshot;
if (nextParent !== prevParent) {
  prevParent.removeChild(this.node);
  nextParent.appendChild(this.node);
}

I'm fairly new to contributing to open-sourced projects. Please let me know if I made any mistakes. Thank you!

---

Acceptance Checklist:

• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• If the commit message has [changed] or [removed], there is an upgrade path above.

[coveralls]

Coverage decreased (-0.2%) to 86.517% when pulling 35aa2ad on husseyexplores:fix-for-#769 into 4ac3ff4 on reactjs:master.

[diasbruno]

Hi @husseyexplores, Thanks for the PR. As mentioned in the issue, you can add a warning telling that the element has gone.

[husseyexplores]

Thanks for the follow-up!
By warning, you mean console.warn, right?

[diasbruno]

Yep. 👍

[husseyexplores]

Added the warning message. Let me know if it needs to be rephrased.

[diasbruno]

Here is an example of a message we are already using https://github.com/reactjs/react-modal/blob/master/src/components/ModalPortal.js#L93.

This should be the last thing.

[diasbruno]

Thanks, @husseyexplores. I'll review it.