ActiveAndroid Model类未正确初始化

我正在尝试使用ActiveAndroid来处理我的应用程序中的数据库.

这是我的模型类:

package ro.adrian.trivia.datamodel;

import java.io.Serializable;

import ro.adrian.trivia.database.TriviaSQLiteHelper;

import com.activeandroid.Model;
import com.activeandroid.annotation.Column;
import com.activeandroid.annotation.Table;

@Table(name = TriviaSQLiteHelper.TABLE_USERS)
public class User extends Model implements Serializable, Comparable{

    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    @Column(name = TriviaSQLiteHelper.COLUMN_USER_NAME)
    private String username;
    @Column(name = TriviaSQLiteHelper.COLUMN_FIRST_NAME)
    private String first_name;
    @Column(name = TriviaSQLiteHelper.COLUMN_LAST_NAME)
    private String last_name;
    @Column(name = TriviaSQLiteHelper.COLUMN_PASSWORD)
    private String password;

    public User(){
        super();
        this.username="";
        this.first_name="";
        this.last_name="";
        this.password="";
    }

    public User(String username, String password, String firstName, String lastName){
        super();
        this.username = username;
        this.password = password;
        this.first_name = firstName;
        this.last_name = lastName;
    }


    public String getFirst_name() {
        return first_name;
    }

    public void setFirst_name(String first_name) {
        this.first_name = first_name;
    }

    public String getLast_name() {
        return last_name;
    }

    public void setLast_name(String last_name) {
        this.last_name = last_name;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    @Override
    public String toString(){
        return this.first_name + " " + this.last_name;
    }


    public String getUsername() {
        return username;
    }


    public void setUsername(String username) {
        this.username = username;
    }

    @Override
    public int compareTo(User arg0) {
        return this.username.compareTo(arg0.username);
    }
}

以下是插入新用户的方法:

public User insertUser(String username, String password, String firstName, String lastName){
        User newUser = new User(username, password, firstName, lastName);
        newUser.save();
        return newUser;
    }

这是我的表现:

在这里,我尝试插入一个新用户:

UserDataSource.getInstance().insertUser("adi", "123", "Adrian", "Olar");

这是logcat:

01-02 14:56:08.818: E/AndroidRuntime(10129): FATAL EXCEPTION: main
01-02 14:56:08.818: E/AndroidRuntime(10129): Process: ro.adrian.trivia, PID: 10129
01-02 14:56:08.818: E/AndroidRuntime(10129): java.lang.NullPointerException
01-02 14:56:08.818: E/AndroidRuntime(10129):    at com.activeandroid.Cache.getTableInfo(Cache.java:146)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at com.activeandroid.Model.(Model.java:50)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at ro.adrian.trivia.datamodel.User.(User.java:36)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at ro.adrian.trivia.database.UserDataSource.insertUser(UserDataSource.java:31)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at ro.adrian.trivia.fragments.LoginFragment$1.onClick(LoginFragment.java:31)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at android.view.View.performClick(View.java:4438)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at android.view.View$PerformClick.run(View.java:18422)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at android.os.Handler.handleCallback(Handler.java:733)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at android.os.Handler.dispatchMessage(Handler.java:95)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at android.os.Looper.loop(Looper.java:136)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at android.app.ActivityThread.main(ActivityThread.java:5017)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at java.lang.reflect.Method.invokeNative(Native Method)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at java.lang.reflect.Method.invoke(Method.java:515)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
01-02 14:56:08.818: E/AndroidRuntime(10129):    at dalvik.system.NativeStart.main(Native Method)

老实说,我不知道我的代码有什么问题......任何建议都非常感谢!