Fix column back button error (#24867)

app/javascript/mastodon/components/column_back_button.jsx

@@ -23,7 +23,7 @@ export default class ColumnBackButton extends React.PureComponent {
       onClick();
     // Check if there is a previous page in the app to go back to per https://stackoverflow.com/a/70532858/9703201
     // When upgrading to V6, check `location.key !== 'default'` instead per https://github.com/remix-run/history/blob/main/docs/api-reference.md#location
-    } else if (router.location.key) {
+    } else if (router.route.location.key) {
       router.history.goBack();
     } else {
       router.history.push('/');