EuiComboBox should not update when not mounted

[timroes]

I succeeded (without knowing exactly how) to trigger the following issue inside EuiComboBox (on the documentation page):

warning.js:33 Warning: Can only update a mounted or mounting component. This usually means you
called setState, replaceState, or forceUpdate on an unmounted component. This is a no-op.

Please check the code for the EuiComboBox component.

Even though I cannot reproduce it, this should be able to be found in code, since it usually means, someone called setState in the resolve of a promise or similar without checking if the component is actually still mounted.

See https://reactjs.org/blog/2015/12/16/ismounted-antipattern.html and facebook/react#2787 (comment)