activity_main.xml选择Design显示rendering problems no render target selected
在stackOverflow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。。。
android studio问题rendering problems no render target selected
原文：http://www.cnblogs.com/jycboy/p/4932016.html