ListView中遇到的空指针异常

public class MainActivity6 extends AppCompatActivity {
    private ListView listView;
    private Vector<News> news = new Vector<>();
    private MyAdapter myAdapter;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main6);
        listView = (ListView) findViewById(R.id.listView5);
        initData();
        myAdapter = new MyAdapter();
        listView.setAdapter(myAdapter);
    }

    private int index = 1;

    /**
     * Init Data
     */


    private void initData() {
        for (int i = 0; i < 10; i++) {

            News n = new News();
            n.title = "title--" + index;
            n.content = "content--" + index;
            index++;
            news.add(n);
        }
    }

    class MyAdapter extends BaseAdapter {


        @Override
        public int getCount() {
            return news.size();
        }

        @Override
        public Object getItem(int position) {
            return news.get(position);
        }

        @Override
        public long getItemId(int position) {
            return position;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            ViewHolder vh;
            if(convertView==null){
                convertView = getLayoutInflater().inflate(R.layout.list_item_6,null);
                vh = new ViewHolder();
                vh.tv_title = (TextView) convertView.findViewById(R.id.textView_title);
                vh.tv_content = (TextView) convertView.findViewById(R.id.textView_content);
                convertView.setTag(vh);
            }else{

                vh = (ViewHolder) convertView.getTag();
            }

            News n = news.get(position);
           vh.tv_title.setText(n.title);
           vh.tv_content.setText(n.content);
            return convertView;
        }

        class ViewHolder {
            TextView tv_title;
            TextView tv_content;
        }
    }
}

异常：
java.lang.NullPointerException
at com.sweetstar86.myapplicationlistview.MainActivity6$MyAdapter.getView(MainActivity6.java:77)
at android.widget.AbsListView.obtainView(AbsListView.java:2622)
at android.widget.ListView.makeAndAddView(ListView.java:1838)
at android.widget.ListView.fillDown(ListView.java:681)
at android.widget.ListView.fillFromTop(ListView.java:742)
at android.widget.ListView.layoutChildren(ListView.java:1659)
at android.widget.AbsListView.onLayout(AbsListView.java:2440)
at android.view.View.layout(View.java:14615)
at android.view.ViewGroup.layout(ViewGroup.java:4491)
at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1021)
at android.view.View.layout(View.java:14615)
at android.view.ViewGroup.layout(ViewGroup.java:4491)
at android.widget.FrameLayout.onLayout(FrameLayout.java:448)
at android.view.View.layout(View.java:14615)
at android.view.ViewGroup.layout(ViewGroup.java:4491)
at android.support.v7.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:435)
at android.view.View.layout(View.java:14615)
at android.view.ViewGroup.layout(ViewGroup.java:4491)
at android.widget.FrameLayout.onLayout(FrameLayout.java:448)
at android.view.View.layout(View.java:14615)
at android.view.ViewGroup.layout(ViewGroup.java:4491)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1669)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1527)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1440)
at android.view.View.layout(View.java:14615)
at android.view.ViewGroup.layout(ViewGroup.java:4491)
at android.widget.FrameLayout.onLayout(FrameLayout.java:448)
at android.view.View.layout(View.java:14615)
at android.view.ViewGroup.layout(ViewGroup.java:4491)
at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2191)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2005)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1218)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4999)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:791)
at android.view.Choreographer.doCallbacks(Choreographer.java:591)
at android.view.Choreographer.doFrame(Choreographer.java:561)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:777)
at android.os.Handler.handleCallback(Handler.java:725)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:5308)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
at dalvik.system.NativeStart.main(Native Method)