fix: Fixes an exception when the chat button and shortcut are clicked.

modules/UI/UI.js

 };
 
 /**
- * Indicates if the filmstrip is currently visible or not.
- * @returns {true} if the filmstrip is currently visible, otherwise
+ * Checks if the filmstrip is currently visible or not.
+ * @returns {true} if the filmstrip is currently visible, and false otherwise.
  */
 UI.isFilmstripVisible = () => Filmstrip.isFilmstripVisible();
 
+/**
+ * @returns {true} if the chat panel is currently visible, and false otherwise.
+ */
+UI.isChatVisible = () => Chat.isVisible();
+
 /**
  * Toggles chat panel.
  */

react/features/toolbox/defaultToolbarButtons.web.js

                 sendAnalytics(createToolbarEvent(
                     'toggle.chat',
                     {
-                        enable: !APP.UI.Chat.isVisible()
+                        enable: !APP.UI.isChatVisible()
                     }));
                 APP.UI.emitEvent(UIEvents.TOGGLE_CHAT);
             },
                 sendAnalytics(createShortcutEvent(
                     'toggle.chat',
                     {
-                        enable: !APP.UI.Chat.isVisible()
+                        enable: !APP.UI.isChatVisible()
                     }));
                 APP.UI.toggleChat();
             },