Warning: Exception thrown by hook while handling onSetChildren: Invariant Violation: Item must have been set

[Matt-Butler]

How to reproduce:

1. Check out the branch modal
2. npm run install && npm run start
3. Navigate to the modal functional examples.
4. Open and close a modal example.
5. Navigate to another page. You should see this warning in the console:

Warning: Exception thrown by hook while handling onSetChildren: Invariant Violation: Item must have been set

[Matt-Butler]

I updated the branch modal-test to demonstrate the issue. I removed all of the code and examples except for the modal.

I have 2 identical modal examples. Only one of the examples throws the warning.

[Matt-Butler]

I am starting to think it is unrelated react-router throwing the warning. I found out another way reproduce the issue.

1. Open and close the first modal.
2. Open the 2nd modal. The warning is in the console.

Warning: Exception thrown by hook while handling onSetChildren: Invariant Violation: Expected onBeforeMountComponent() parent and onSetChildren() to be consistent (28 has parents 22 and 27).
Invariant Violation: Expected onBeforeMountComponent() parent and onSetChildren() to be consistent (28 has parents 22 and 27).

[Matt-Butler]

facebook/react#8692

facebook/react#8692 (comment)

[Matt-Butler]

We tried out the example above using react 16.0.0-alpha.11. We could not reproduce the issue with react 16.0.0-alpha.11.

[Matt-Butler]

I think we will go forward with this pull request expecting the update to react 16.x to fix the issue:
#224