SlidePager | ViewPagerGridView实现图片滑动

Attempt to invoke virtual method 'void androidx.viewpager.widget.ViewPager.setAdapter(androidx.viewpager.widget.PagerAdapter)' on a null object reference

I'm trying to implement sliding tab layout using android material design. But it gives me NullPointerException. Here is my code so far:

public class HomeFragment extends Fragment {

I made a custom pager for my bxSlider. I want to change the original grey img to the colored image when it is clicked and get class "green". While the clicked pager has class "green" and colored image, I want the other pager stay as the original gray image.

I got to the part where I can replace the original gray img to green, and remove the class "green" of the siblings, however, even though the class "green" is removed from the siblings, it still remains as colored img instead of going back to the original grey as if removeClass("green") is not working.

HTML: