fix EuiSuperSelect from accessing ref when unmounted

[nreese]

Summary

The kibana GIS app places an EuiSuperSelect in a FlexItem that is unmounted when the user clicks cancel (first screen shot). If the menu is open when the cancel button is clicked, the EuiSuperSelect will be unmounted but focusSelected gets called afterward, via requestAnimationFrame, and this causes a hard crash (second screen shot, third error message). This PR just adds a check to ensure the component is still mounted before proceeding with the contents of the requestAnimationFrame callback.

Checklist

• This was checked in mobile
• This was checked in IE11
• This was checked in dark mode
• Any props added have proper autodocs
• Documentation examples were added
• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately
• Jest tests were updated or added to match the most common scenarios
• This was checked against keyboard-only and screenreader scenarios
• This required updates to Framer X components

[chandlerprall]

change LGTM

[cchaos]

👍