activity_main.xml选择Design显示rendering problems no render target selected

在*上找到了答案：

You need to run the AVD Manager to add a virtual device that matches the target API version your project is set to.

Click Tools->Android->AVD Manager. Click on 'New', add virtual devices targeting higher API levels, e.g 18, 19, etc.

Then restart Android Studio, and open your project, it should find a target device to allow you access to the Design view.

就是新建一个虚拟机，api选择高点，我选的21，重启成功。。。