问getReadableDatabase上的android指针异常

EN

public static synchronized MetroLinkDatabaseHelper getInstance(Context context) {

//        Using a singleton to minmize the chance of opening multiple
//        decreasing any chance of memory leak
        if(myInstance == null) {
            myInstance = new MetroLinkDatabaseHelper(context);
        }
        return myInstance;
    }
  private MetroLinkDatabaseHelper(Context context) {
        super(context, DB_NAME, null, 1);
        this.mycontext = context;
        boolean dbexist = checkdatabase();
        if (dbexist) {
        } else {
            System.out.println("Database doesn't exist");
            try {
                createdatabase();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }

    //      Create (copy the original in assets) if not exist using copydatabase
    public void createdatabase() throws IOException {
        boolean dbexist = checkdatabase();
        if (dbexist) {
        } else {
//            SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
             this.getReadableDatabase();
                 try {

                copydatabase();
            } catch (IOException e) {
                throw new Error("Error copying database");
            }
        }
    }

08-29 14:13:05.147    2222-2222/com.bkane56.practice.practiceapp E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.bkane56.practice.practiceapp, PID: 2222
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.bkane56.practice.practiceapp/com.bkane56.practice.practiceapp.ListStopsActivity}: 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:2250)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2413)
            at android.app.ActivityThread.access$800(ActivityThread.java:155)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1317)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5356)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:905)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:700)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
            at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:268)
            at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:223)
            at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:187)
            at com.bkane56.practice.practiceapp.MetroLinkDatabaseHelper.createdatabase(MetroLinkDatabaseHelper.java:66)
            at com.bkane56.practice.practiceapp.MetroLinkDatabaseHelper.<init>(MetroLinkDatabaseHelper.java:52)
            at com.bkane56.practice.practiceapp.MetroLinkDatabaseHelper.getInstance(MetroLinkDatabaseHelper.java:38)
            at com.bkane56.practice.practiceapp.ListStopsActivity.<init>(ListStopsActivity.java:28)
            at java.lang.reflect.Constructor.newInstance(Native Method)
            at java.lang.Class.newInstance(Class.java:1606)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1089)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2240)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2413)
            at android.app.ActivityThread.access$800(ActivityThread.java:155)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1317)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5356)
            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:905)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:700)

this.getReadableDatabase();