null object reference

[spydermard]

hi.
I'm building new project by sample project but I have an error that can not solve it.

MainActivity:
`
package com.example.alireza.myapplication;

import android.graphics.Color;
import android.graphics.drawable.ColorDrawable;
import android.graphics.drawable.Drawable;
import android.graphics.drawable.LayerDrawable;
import android.graphics.drawable.TransitionDrawable;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.content.ContextCompat;
import android.support.v4.view.ViewPager;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.widget.Toolbar;
import android.util.TypedValue;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Toast;

import com.astuetz.PagerSlidingTabStrip;
import com.readystatesoftware.systembartint.SystemBarTintManager;

import butterknife.BindView;
import butterknife.ButterKnife;

public class MainActivity extends ActionBarActivity {

@BindView(R.id.toolbar)
Toolbar toolbar;
@BindView(R.id.tabs)
PagerSlidingTabStrip tabs;
@BindView(R.id.pager)
ViewPager pager;

private MyPagerAdapter adapter;
private Drawable oldBackground = null;
private int currentColor;
private SystemBarTintManager mTintManager;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ButterKnife.bind(this);
    setSupportActionBar(toolbar);
    // create our manager instance after the content view is set
    mTintManager = new SystemBarTintManager(this);
    // enable status bar tint
    mTintManager.setStatusBarTintEnabled(true);
    adapter = new MyPagerAdapter(getSupportFragmentManager());

    pager.setAdapter(adapter);
    tabs.setViewPager(pager);
    final int pageMargin = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 4, getResources()
            .getDisplayMetrics());
    pager.setPageMargin(pageMargin);
    pager.setCurrentItem(1);
    changeColor(ContextCompat.getColor(getBaseContext(), R.color.green));

    tabs.setOnTabReselectedListener(new PagerSlidingTabStrip.OnTabReselectedListener() {
        @Override
        public void onTabReselected(int position) {
            Toast.makeText(MainActivity.this, "Tab reselected: " + position, Toast.LENGTH_SHORT).show();
        }
    });
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case R.id.action_contact:
            QuickContactFragment.newInstance().show(getSupportFragmentManager(), "QuickContactFragment");
            return true;
    }
    return super.onOptionsItemSelected(item);
}

private void changeColor(int newColor) {
    tabs.setBackgroundColor(newColor);
    mTintManager.setTintColor(newColor);
    // change ActionBar color just if an ActionBar is available
    Drawable colorDrawable = new ColorDrawable(newColor);
    Drawable bottomDrawable = new ColorDrawable(ContextCompat.getColor(getBaseContext(), android.R.color.transparent));
    LayerDrawable ld = new LayerDrawable(new Drawable[]{colorDrawable, bottomDrawable});
    if (oldBackground == null) {
        getSupportActionBar().setBackgroundDrawable(ld);
    } else {
        TransitionDrawable td = new TransitionDrawable(new Drawable[]{oldBackground, ld});
        getSupportActionBar().setBackgroundDrawable(td);
        td.startTransition(200);
    }

    oldBackground = ld;
    currentColor = newColor;
}

public void onColorClicked(View v) {
    int color = Color.parseColor(v.getTag().toString());
    changeColor(color);
}

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putInt("currentColor", currentColor);
}

@Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
    super.onRestoreInstanceState(savedInstanceState);
    currentColor = savedInstanceState.getInt("currentColor");
    changeColor(currentColor);
}

public class MyPagerAdapter extends FragmentPagerAdapter {

    private final String[] TITLES = {"Categories", "Home", "Top Paid", "Top Free", "Top Grossing", "Top New Paid",
            "Top New Free", "Trending"};

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

    @Override
    public CharSequence getPageTitle(int position) {
        return TITLES[position];
    }

    @Override
    public int getCount() {
        return TITLES.length;
    }

    @Override
    public Fragment getItem(int position) {
        return SuperAwesomeCardFragment.newInstance(position);
    }
}

}`

my error message:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.alireza.myapplication/com.example.alireza.myapplication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2434) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2494) at android.app.ActivityThread.access$900(ActivityThread.java:157) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1356) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5551) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.view.ViewPager.setAdapter(android.support.v4.view.PagerAdapter)' on a null object reference at com.example.alireza.myapplication.MainActivity.onCreate(MainActivity.java:69) at android.app.Activity.performCreate(Activity.java:6272) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2387) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2494) at android.app.ActivityThread.access$900(ActivityThread.java:157) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1356)  at android.os.Handler.dispatchMessage(Handler.java:102)  at android.os.Looper.loop(Looper.java:148)  at android.app.ActivityThread.main(ActivityThread.java:5551)  at java.lang.reflect.Method.invoke(Native Method)  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620)