我按照以下步骤解决了这个问题：

I solved it following these steps:

1. 打开Xcode 6 beta


2. 进入菜单Xcode>打开开发人员工具> iOS模拟器


3. 即使出现错误对话框，您仍然可以访问iOS模拟器菜单


4. 选择硬件>设备>管理设备


5. 点击底部的小+号


6. 添加（如果缺少）您想要的所有设备，或者删除并重新创建发生故障的人。


7. 如果模拟器中的任何人不能正常工作，请右键单击并删除它然后重新创建

1. Open Xcode 6 beta
2. Go to the menu Xcode > Open Developer Tool > iOS Simulator
3. Even if an error dialog shows up, you still would have access to the iOS Simulator's menu
4. Select Hardware > Device > Manage Devices
5. Click on the little + sign at the bottom
6. Add (if missing) all the devices that you want, or delete and recreate the ones malfunctioning.
7. If anyone of the simulator is not working then right click on it and delete it and then re-create it again