我编写了一个方法（verifyUser）来检查用户是否已经存在于数据库中，但是出现此错误：

  

（java.lang.NullPointerException：尝试调用虚拟方法   'android.database.sqlite.SQLiteDatabase   com.example.testapp.DatatBaseHelper.getReadableDatabase（）'为null   对象参考）

那是我的ContentProvider类：

public class TestProvider extends ContentProvider {

    private DatatBaseHelper helper;
    private SQLiteDatabase db;
    public static final String LOG_TAG = TestProvider.class.getSimpleName();
    private static final int test=1;
    private static final int test_id=2;

    private static final UriMatcher urim=new UriMatcher(UriMatcher.NO_MATCH);

    static{
        urim.addURI(Test.test.authority,Test.test.TABLE_NAME,test);
        urim.addURI(Test.test.authority,Test.test.TABLE_NAME+"/#",test_id);
    }
    @Override
    public boolean onCreate() {
        helper=new DatatBaseHelper(getcontext());
        return false;
    }

    @Nullable
    @Override
    public Cursor query(@NonNull Uri uri,@Nullable String[] projection,@Nullable String selection,@Nullable String[] selectionArgs,@Nullable String sortOrder) {
        int match=urim.match(uri);

      db=helper.getReadableDatabase();

        Cursor c;

        switch(match){
            case test:
                c=db.query(Test.test.TABLE_NAME,projection,selection,selectionArgs,null,sortOrder);
                break;

            case test_id:
                selection= Test.test.ID+"=?";
                selectionArgs=new String[]{String.valueOf(Contenturis.parseId(uri))};
                c=db.query(Test.test.TABLE_NAME,sortOrder);

                break;
            default:
                throw new IllegalStateException("Cannot query form: " + uri);
        }

        return null;
    }

    @Nullable
    @Override
    public String getType(@NonNull Uri uri) {
        return null;
    }

    @Nullable
    @Override
    public Uri insert(@NonNull Uri uri,@Nullable Contentvalues values) {

         db=helper.getwritabledatabase();

        String email=values.getasString(Test.test.EMAIL);





        int match=urim.match(uri);

        switch (match){
            case test:
          int   id=(int)(db.insert(Test.test.TABLE_NAME,values));




                uri= Contenturis.withAppendedId(uri,id);
        }


        return uri;
    }

    @Override
    public int delete(@NonNull Uri uri,@Nullable String[] selectionArgs) {
        return 0;
    }

    @Override
    public int update(@NonNull Uri uri,@Nullable Contentvalues values,@Nullable String[] selectionArgs) {
        return 0;
    }


    public boolean verifyUser(String email){

        SQLiteDatabase db=helper.getReadableDatabase();

        Cursor c;

        String pro[]={Test.test.ID};
        String select=Test.test.EMAIL+"=?";
        String args[]={email};

        c=db.query(Test.test.TABLE_NAME,pro,select,args,null);

        if(c.getcount()>0){
            return true;
        }

        return false;
    }

那是我的数据库类：

public class DatatBaseHelper extends SQLiteOpenHelper {

    public static final  String DB_NAME="dress.db";
    public static final int DB_VERSION=1;

    public DatatBaseHelper(Context c){
        super(c,DB_NAME,DB_VERSION);
    }


    private  String TABLE_CREATE= "CREATE TABLE " + test.TABLE_NAME+ "( "+
  test. ID+" INTEGER PRIMARY KEY AUTOINCREMENT,"+ test.NAME+" TEXT NOT NULL,"+
   test. EMAIL+" TEXT NOT NULL,"+ test.PASSWORD+ " TEXT NOT NULL);";



        @Override
        public void onCreate(SQLiteDatabase db) {
    db.execSQL(TABLE_CREATE);
        }

        @Override
        public void onUpgrade(SQLiteDatabase db,int oldVersion,int newVersion) {

        }






    }