Android Studio no render target selected

Today for the first time using Android studio, inexplicably appeared no render target selected error, no design interface, various Baidu after stackoverflow.com/questions/22776653/ No-rendering-target-selected-using-android-studio found the answer, I solved it with File Invalidate Caches/Restart . Although the solution is still inexplicable, guess the reason may be, After installing Android Studio did not create the AVD first, and did not open the genymotion caused by the problem, because all with genymotion to replace the IDE's own simulator, so ignore this step, after the completion of the above two steps, the problem is resolved.

Another problem is that api21,api18 File Invalidate Caches/Restart only appeared after.

Android Studio no render target selected