fix(FocusZone): Cleanup DOM element references on unmount

CHANGELOG.md

@@ -35,6 +35,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Fix `forceUpdate` to get synced updates in React's Concurrent mode @layershifter ([#2268](https://github.com/microsoft/fluent-ui-react/pull/2268))
 - Adding actionable items into `Carousel` @kolaps33 ([#2271](https://github.com/microsoft/fluent-ui-react/pull/2271))
 - Fix positioning fixes for `actions` in `ChatMessage` @layershifter ([#2300](https://github.com/microsoft/fluent-ui-react/pull/2300))
+- `FocusZone`: Cleanup DOM element references when the component is unmounted @johannao76 ([#2306](https://github.com/microsoft/fluent-ui-react/pull/2306))
 
 ### Features
 - Allow `useRef` hook used for storing debugging data to be defined in any order with other hooks in functional components @layershifter, @mnajdova ([#2236](https://github.com/microsoft/fluent-ui-react/pull/2236))

packages/react-bindings/src/FocusZone/FocusZone.tsx

@@ -206,6 +206,9 @@ export default class FocusZone extends React.Component<FocusZoneProps> implement
     if (this._root.current) {
       this._root.current.removeEventListener('blur', this._onBlur, true)
     }
+
+    this._activeElement = null
+    this._defaultFocusElement = null
   }
 
   render() {