带有空对象引用的抛出错误

Question

listView1.setAdapter（adapter）上的以下错误，带有setApdater的空对象引用。我想第二列索引设置为null。

historyActivity.java

public class historyActivity extends MainActivity {
    private TextView mTextMessage;
    Intent intent = new Intent();

    DatabaseHelper myDB;

    ArrayList<User> userList;
    ListView listView1;
    User user;
    ThreeColumn_ListAdapter adapter;

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

        myDB = new DatabaseHelper(this);

        userList = new ArrayList<>();
        Cursor data = myDB.getListContents();
        int numRows = data.getCount();
        if(numRows ==0){
            Toast.makeText(historyActivity.this, "Nothing in the db",Toast.LENGTH_LONG).show();
        }
        else{
            int i = 0;
            while(data.moveToNext()){
                user = new User(data.getString(1), data.getString(2), data.getString(3));
                userList.add(user);
            }
            adapter = new ThreeColumn_ListAdapter(this,R.layout.viewcontents_layout,userList);
            listView1 = (ListView)findViewById(R.id.listView);
            listView1.setAdapter(adapter);

        }

    }


}