How to use Intent() to open the viewpager activity

Question

This is my idea: If i press the button, current page which is main_activity will change to viewpager_activity. I use "intent()" order to change to viewpager_activity. My viewpager has 3 tabs: green-red-yellow (in order). However after i press the button, the app has problem and appears " unfortunately my application has stopped" Please help me!

this's my main_activity

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.ImageButton;

public class MainActivity extends AppCompatActivity {
    ImageButton btn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        btn = (ImageButton)findViewById(R.id.btn_health);

        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent mh_viewpager = new Intent(MainActivity.this, Viewpager.class);
                startActivity(mh_viewpager);
            }
        });
    }
}

this's my viewpager

package dogcaringsystem.hcmut.baotoan.caringdog;

import android.graphics.Color;
import android.os.Bundle;
import android.support.constraint.ConstraintLayout;
import android.support.design.widget.TabLayout;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class Viewpager extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_viewpager);

        Toolbar toolbar = (Toolbar)findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        SectionsPagerAdapter mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

        ViewPager mViewPager = (ViewPager) findViewById(R.id.viewpager);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        TabLayout tabLayout = (TabLayout)findViewById(R.id.tabs);
        tabLayout.setupWithViewPager(mViewPager);
    }

    public static class PlaceholderFragment extends Fragment {

        private static final String KEY_COLOR = "key_color";

        public PlaceholderFragment() {
        }

        public static PlaceholderFragment newInstance(int color_xml) {
            PlaceholderFragment fragment = new PlaceholderFragment();
            Bundle args = new Bundle();
            args.putInt(KEY_COLOR, color_xml);
            fragment.setArguments(args);
            return fragment;
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.activity_viewpager, container, false);
            ConstraintLayout constraintLayout = (ConstraintLayout) rootView.findViewById(R.id.fragment);

            /*
             * num 1: green.
             * num 2: red.
             * num 3: yellow.
             */
            switch (getArguments().getInt(KEY_COLOR)) {
                case 1:
                    constraintLayout.setBackgroundColor(Color.GREEN);
                    break;
                case 2:
                    constraintLayout.setBackgroundColor(Color.RED);
                    break;
                case 3:
                    constraintLayout.setBackgroundColor(Color.YELLOW);
                    break;
                default:
                    constraintLayout.setBackgroundColor(Color.GREEN);
                    break;
            }

            return rootView;
        }
    }

    private class SectionsPagerAdapter extends FragmentPagerAdapter {

        SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            // position + 1 because position start at 0.
            return PlaceholderFragment.newInstance(position + 1);
        }

        @Override
        public int getCount() {
            return 3;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            switch (position) {
                case 0:
                    return "section 1";
                case 1:
                    return "section 2";
                case 2:
                    return "section 3";
            }
            return null;
        }
    }
}