Android : Null Pointer Exception when setting button visibility

I am getting to change the visibility of one of the buttons on my viewpager but I am getting a Null Pointer Exception on this line:

save_button.setVisibility(Button.INVISIBLE);

I am wondering why that is? Is it because I am not getting the visibility property inside of my InstantiateItem() for the button? Should I? If so, how do I change the button visibility based on the fact if the page is viewed by the user or not?

What I am trying to do is: Show the save button if all the views are viewed in the viewpager. If not all the views are viewed, then hide the save button.

Here is my code:

boolean isViewed = false;
                boolean buttonState= false; //unpressed, if true == pressed
                int buttonValue = 0;

        //Methods
        protected void onCreate(Bundle savedInstanceState) {

            super.onCreate(savedInstanceState);
            setContentView(R.layout.score_viewpager);

            //Specify the number of pages/views
            numberOfPages = new Integer[]{1, 2, 3, 4, 5};

            final Button save_button = (Button) findViewById(R.id.save);

            //Initialize adapter to populate view
            myAdapter = new MyAdapter(ScoreCollectionPager.this, numberOfPages, save_button);

            //Search view for viewpager Id and set the adapter on the first item
            viewPager = (ViewPager) findViewById(R.id.viewPager);
            viewPager.setAdapter(myAdapter);
            viewPager.setCurrentItem(0);

            //Attach the page change listener inside the activity
            viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {

                Button save_button = (Button) findViewById(R.id.save);

                // This method will be invoked when the current page is scrolled
                @Override
                public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

                }

                //This method will be invoked when a new page becomes selected
                @Override
                public void onPageSelected(int position) {
                    //get position
                    isViewed = true;

                   if (isViewed  && (position == numberOfPages.length)) {

                        save_button.setVisibility(Button.VISIBLE);
                    }
                    else {
                        save_button.setVisibility(Button.INVISIBLE); // Null Pointer Exception here? why?
                    }

                }

                // Called when the scroll state changes:
                // SCROLL_STATE_IDLE, SCROLL_STATE_DRAGGING, SCROLL_STATE_SETTLING
                @Override
                public void onPageScrollStateChanged(int i) {
                    //get state
                }
            });

        }

                 private class MyAdapter extends PagerAdapter {

                //fields
                Button save_button;

                //Constructor
                public MyAdapter(Context context, numberOfPages, Button save_button) {

                    this.context = context;
                    this.numberOfPages = new ArrayList<Integer>(Arrays.asList(numberOfPages));
                    this.save_button = save_button;

                }

                //Returns total number of pages
                @Override
                public int getCount() {
                    return numberOfPages.size();
                }

                /**
                 * Create the page for the given position.
                 *
                 * @param parent The containing View in which the page will be shown.
                 * @param position  The page position to be instantiated.
                 *
                 * @return Returns an Object representing the new page. This does not need
                 *         to be a View, but can be some other container of the page.
                 */
                @Override
                public Object instantiateItem(ViewGroup parent, final int position) {


                    //Get the inflater
                    LayoutInflater inflater = (LayoutInflater) parent.getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);

                    //inflate the root layout
                    View view = inflater.inflate(R.layout.score_collection, null);

                                        //Save Button
                    save_button = (Button) view.findViewById(R.id.save);

                    save_button.setOnClickListener(new ViewGroup.OnClickListener() {
                        public void onClick(View view) {

                                                //code for saving

                                                }});