Presenting view controllers on detached view controllers is discouraged

出现这个警告的原因是presentViewController的receiver是一个detached view controller。比方说：window有一个root view controller，然后我们把某个view controller(VC1)的view添加到了root view controller的subview上面，然后在VC1上面presentViewController。这时我们需要将VC1添加到root view controller的childController里面去。