Accordion: Mutation observer is not removed

[w33ble]

See https://github.com/elastic/eui/blob/master/src/components/accordion/accordion.js#L67-L70

A mutation observer is created when the Accordion renders, but it is never removed. When the component unmounts, setChildContentHeight can still be called, and would result in an error:

It's not clear from the MDN docs, but this could also result in a memory leak if the listener isn't automatically cleaned when the DOM node is removed.

[w33ble]

This is a non-issue. The ref function is called with null, which effectively removes the observer without creating a new one. Sorry for the noise.