In this code, the OnClick of next button is written on the implementation of onItemSelected,so If a user clicks the next button without any clicks on the images of grid view then it doesn't take any actions ,to ensure that onclickListener must be written in oncreate function of the main activity.
https://github.com/udacity/Android_Me/blob/TFragments.06-Solution-CommunicateBetweenActivities/app/src/main/java/com/example/android/android_me/ui/MainActivity.java
In this code, the OnClick of next button is written on the implementation of onItemSelected,so If a user clicks the next button without any clicks on the images of grid view then it doesn't take any actions ,to ensure that onclickListener must be written in oncreate function of the main activity.