Bug - AlertGroup - li elements never unmount for timedout alerts

Describe the problem
When Alerts with a timeout are rendered inside an AlertGroup, the wrapper li element for the Alerts never unmounts when the Alerts do. This causes empty li elements to remain in the DOM.

How do you reproduce the problem?
Go to the alert timeout example and add an alert in the example. Inspect the DOM and notice that when the Alert itself unmounts, the wrapper li element remains with nothing inside of it. Add another Alert and notice that the empty li still remains in addition to a new li with the new Alert rendered inside of it.

Expected behavior
The li elements would unmount the same as the Alerts themselves

Is this issue blocking you?
List the workaround if there is one.

Screenshots
If applicable, add screenshots to help explain the issue.

What is your environment?

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

What is your product and what release date are you targeting?

Any other information?

AlertGroupコンポーネントから始め、alert timeoutの例を使って問題を再現します。タイムアウトしたAlertsが消える際のDOMを調べ、それらをラップするli要素がどのようにレンダリングされ、削除されるかを追跡します。別のAlertが追加された場合も含め、タイムアウトしたAlertsが空のli要素を残さなくなれば完了です。