Describe the bug

The isOpen prop added in #8896 introduced a bug triggered when the new optional prop is not defined. isOpen is an optional prop and the component should behave as previously when the prop is omitted.

In case this isn't easy to solve, we should remove the isOpen prop from the flyout system feature branch and introduce it separately if needed.

Impact and severity

This issue exists in feat/flyout-system feature branch and isn't production facing.

Environment and versions

All

To Reproduce

1. Run the latest feat/flyout-system branch locally
2. Open http://localhost:6006/?path=/story/layout-euiflyout-flyout-manager--multi-session-example
3. Update the story to use conditional rendering instead of the new isOpen prop and notice the difference.

Expected behavior

Managed flyouts (session={true}) should behave the same no matter whether the isOpen prop is defined or not.