Check if router is injected in media modal (#7941)

app/javascript/mastodon/features/ui/components/media_modal.js

@@ -67,21 +67,25 @@ export default class MediaModal extends ImmutablePureComponent {
 
   componentDidMount () {
     window.addEventListener('keyup', this.handleKeyUp, false);
+    if (this.context.router) {
       const history = this.context.router.history;
       history.push(history.location.pathname, previewState);
       this.unlistenHistory = history.listen(() => {
         this.props.onClose();
       });
     }
+  }
 
   componentWillUnmount () {
     window.removeEventListener('keyup', this.handleKeyUp);
+    if (this.context.router) {
       this.unlistenHistory();
 
       if (this.context.router.history.location.state === previewState) {
         this.context.router.history.goBack();
       }
     }
+  }
 
   getIndex () {
     return this.state.index !== null ? this.state.index : this.props.index;