Remove event listeners for Location on unmount

Currently, if the component is not mounted (e.g. because on a different page),
the component will still receive the change event and the `onLocationChange`
callback changes its props via `this.props.setProps` with the itempath from
('_dashprivate_path') in the DOM. The object, however, is
not on the dom and a random other DOM element is assigned the changed props.

The PR deregisters the event listeners of the component is unmounted. They will
be added again via `componentDidMount` anyways if the component is mounted again.

fixes plotly#1346

components/dash-core-components/src/components/Location.react.js

@@ -121,6 +121,14 @@ export default class Location extends Component {
         this.updateLocation(this.props);
     }
 
+    componentWillUnmount() {
+        window.onpopstate = () => {};
+        window.removeEventListener(
+            '_dashprivate_pushstate',
+            this.onLocationChange
+        );
+    }
+
     UNSAFE_componentWillReceiveProps(nextProps) {
         this.updateLocation(nextProps);
     }