View使用相关错误 —— 吃一堑，长一智

java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View androidx.viewpager.widget.ViewPager.findViewById(int)' on a null object reference

Attempt to invoke interface method 'boolean java.util.List.add(java.lang.Object)' on a null object reference

只有声明