Remove created event listeners on unmount

In order to avoid memory leaks, component **should clean after itself** and call `removeEventListener` for every listener declared on `componentDidMount`.

See [related official docs](https://react-cn.github.io/react/tips/dom-event-listeners.html).

Start by locating the componentDidMount implementation and inventory the event listeners declared there. Read the linked React DOM event-listener guidance, then verify that unmount cleanup covers every listener and that the component no longer retains them after unmount.

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