运行时异常:SQLite数据库出现NullPointerException

我有一个带有谷歌地图和标记的Andorid应用程序。如果你点击一个标记，你就会进入位置的"主屏幕"，在那里你可以用按钮开始测试。完成5道选择题后，显示分数(1-5分(。到目前为止，一切都在起作用。现在，我想将分数保存在SQLite数据库中，如果分数为5，则在"主屏幕"上更改图像。现在，每次我测试并点击标记打开特定位置的"主屏幕"(如果分数=5，则应更改图像(时，应用程序都会崩溃。到目前为止，表中还没有数据，因为我从未通过测验来保存分数。如果有人发现错误，我将不胜感激！

这是我的代码：

数据库帮助程序：

public class DbHelper extends SQLiteOpenHelper {
private static final int DATABASE_VERSION = 7;
private static final String DATABASE_NAME = "CE";
public static final String SCORE_TABLE = "score";
public static final String COLUMN_ID = "ID";
public static final String COLUMN_SCORE = "SCORE";
public static final String COLUMN_MARKERID = "MARKERID";
 private SQLiteDatabase dbase;
 public DbHelper(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
    //Hier alle Tables erstellen
    String create_query = "CREATE TABLE IF NOT EXITS " + SCORE_TABLE + " ( "
            + COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, "
            + COLUMN_SCORE + " INTEGER, "
            + COLUMN_MARKERID + " TEXT) ";
    db.execSQL(create_query);
}
public void addScore (DbHelper dbh, Integer score, String markerID) {
    dbase = dbh.getWritableDatabase();
    ContentValues cv = new ContentValues();
    cv.put(COLUMN_SCORE, score);
    cv.put(COLUMN_MARKERID, markerID);
    dbase.insert(SCORE_TABLE, null, cv);
}
public Cursor getScore(DbHelper dbh) {
    dbase = dbh.getReadableDatabase();
    String columns[] = {COLUMN_SCORE, COLUMN_MARKERID};
    Cursor cursor = dbase.query(SCORE_TABLE, columns, null, null, null, null, null);
    return cursor;
}

在ResultActivity:中完成测验后，将分数写入数据库

public class ResultActivity extends Activity {
String markerID;
int score;
TextView t=(TextView)findViewById(R.id.textResult);
Button saveButton = (Button) findViewById(R.id.saveButton);
Context context = this;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.result_layout);
    Bundle b = getIntent().getExtras();
    score = b.getInt("score");
    markerID = b.getString("markerID");
}
saveButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            DbHelper dbh = new DbHelper(context);
            dbh.addScore(dbh,score,markerID);
        }
    });
}

看看"主屏幕"，这个标记上保存了什么分数：

public class Discover extends ActionBarActivity {
TextView InfoHeadline;
ImageView ImageDone;
Button QuizButton;
String markerID;
Context context;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.discover_layout);
    InfoHeadline = (TextView)findViewById(R.id.InfoUeberschrift);
    ImageDone =(ImageView)findViewById(R.id.imageDone);
    QuizButton = (Button)findViewById(R.id.QuizButton);
//get markerID from previous google maps activity
    if (savedInstanceState == null) {
        Bundle extras = getIntent().getExtras();
        if(extras == null) {
            markerID= null;
        } else {
            markerID= extras.getString("MarkerID");
        }
    } else {
        markerID = (String) savedInstanceState.getSerializable("MarkerID");
    }
    InfoHeadline.setText(markerID);
    DbHelper dbh  = new DbHelper(context);
    Cursor cursor = dbh.getScore(dbh);
    cursor.moveToFirst();
    do {
        if (Integer.parseInt(cursor.getString(0))== 5 && InfoHeadline.toString().equals(cursor.getString(1))){
            ImageDone.setImageResource(R.drawable.markerdone);
        }
    }while(cursor.moveToNext());
}
public void StartQuiz (View v) {
    Intent intent = new Intent(Discover.this,QuizActivity.class);
    intent.putExtra("MarkerID", markerID);
    startActivity(intent);
}
}

这是坠机报告：

java.lang.RuntimeException: Unable to start activity ComponentInfo{de.marmor.discover/de.marmor.discover.Discover}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2345)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2407)
        at android.app.ActivityThread.access$800(ActivityThread.java:149)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1324)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:211)
        at android.app.ActivityThread.main(ActivityThread.java:5321)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1016)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:811)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
        at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:223)
        at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:187)
        at de.marmor.discover.DbHelper.getScore(DbHelper.java:71)
        at de.marmor.discover.Discover.onCreate(Discover.java:46)
        at android.app.Activity.performCreate(Activity.java:5933)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)`

 DbHelper dbh  = new DbHelper(context);

 DbHelper dbh  = new DbHelper(getApplicationContext());