Fix react warnings and part of backstage (#4706)

webapp/components/user_settings/user_settings_modal.jsx

@@ -77,16 +77,24 @@ class UserSettingsModal extends React.Component {
         };
 
         this.requireConfirm = false;
+        this.mounted = false;
     }
 
     onUserChanged() {
-        this.setState({currentUser: UserStore.getCurrentUser()});
+        if (this.mounted) {
+            this.setState({currentUser: UserStore.getCurrentUser()});
+        }
     }
 
     componentDidMount() {
+        this.mounted = true;
         UserStore.addChangeListener(this.onUserChanged);
     }
 
+    componentWillUnmount() {
+        this.mounted = false;
+    }
+
     componentDidUpdate() {
         UserStore.removeChangeListener(this.onUserChanged);
         if (!Utils.isMobile()) {