Не инициализируется вспомогательный класс наследующий SQLiteOpenHelper

Не инициализируется вспомогательный класс наследующий SQLiteOpenHelper, от чего выдает NullPointerException. Код БД

public class DB {
    private static final String DB_NAME = "taskDB";
    private static final int DB_VERSION = 1;
    private static final String TABLE_STUDY_TASK = "studyTable";
    private static final String KEY_ID = "_id";

    public static final String KEY_SUBJECT_NAME = "subName";
    public static final String KEY_OBJECTIVE_NAME = "objName";
    public static final String KEY_TIME_OF_TASK = "taskTime";
    public static final String KEY_TASK_IMAGE = "taskImg";



    private static final String CREATE_TABLE_STUDY =
            "create table if not exists " + TABLE_STUDY_TASK + "(" + KEY_ID + " integer primary key autoincrement, " + KEY_SUBJECT_NAME + " text, " + KEY_OBJECTIVE_NAME + " text, " +
            KEY_TASK_IMAGE + " integer, " + KEY_TIME_OF_TASK + " numeric" + ");";

    private final Context mCtx;

    private DBHelper mDBHelper;
    private SQLiteDatabase mDB;

    public DB(Context Ctx) {
        mCtx = Ctx;
    }


    public void open(){
        mDBHelper = new DBHelper(mCtx, DB_NAME, null, DB_VERSION);
        mDB = mDBHelper.getWritableDatabase();
    }

    public void close(){
        if(mDBHelper != null) mDBHelper.close();
    }

    public Cursor getTaskStudyData(){
        return mDB.query(TABLE_STUDY_TASK, null, null, null, null, null, null);
    }

    public void addRecToStudyTable(String subject_name, String objective_name, int image, String time ){

        ContentValues cv = new ContentValues();
        cv.put(KEY_SUBJECT_NAME, subject_name);
        cv.put(KEY_OBJECTIVE_NAME, objective_name);
        cv.put(KEY_TASK_IMAGE, image);
        cv.put(KEY_TIME_OF_TASK, time);
        mDB.insert(TABLE_STUDY_TASK, null, cv);
    }

    public void delRecStudyTable(long id){
        mDB.delete(TABLE_STUDY_TASK, KEY_ID + " = " + id, null);
    }


    private class DBHelper extends SQLiteOpenHelper {

        public DBHelper(Context context, String name, SQLiteDatabase.CursorFactory factory,
                        int version) {
            super(context, name, factory, version);
        }


        // создаем и заполняем БД
        @Override
        public void onCreate(SQLiteDatabase db) {
            db.execSQL(CREATE_TABLE_STUDY);
            ContentValues cv = new ContentValues();
            for(int i = 0; i<5; i++){
                cv.put(KEY_SUBJECT_NAME, "someSubject" + i);
                cv.put(KEY_OBJECTIVE_NAME, "someObj" + i);
                cv.put(KEY_TASK_IMAGE, R.drawable.ic_launcher_background);
                cv.put(KEY_TIME_OF_TASK, "11:00");
                mDB.insert(TABLE_STUDY_TASK, null, cv);
            }

        }

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

        }
    }
}

Лог ошибки

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.checklist, PID: 17046
    java.lang.NullPointerException: Attempt to invoke virtual method 'long android.database.sqlite.SQLiteDatabase.insert(java.lang.String, java.lang.String, android.content.ContentValues)' on a null object reference
        at com.example.checklist.DB$DBHelper.onCreate(DB.java:82)
        at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:393)
        at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:298)
        at com.example.checklist.DB.open(DB.java:38)
        at com.example.checklist.FirstTypeTaskFragment.onCreateView(FirstTypeTaskFragment.java:56)
        at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2698)
        at androidx.fragment.app.FragmentStateManager.createView(FragmentStateManager.java:310)
        at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1185)
        at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1354)
        at androidx.fragment.app.FragmentManager.moveFragmentToExpectedState(FragmentManager.java:1432)
        at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1495)
        at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:447)
        at androidx.fragment.app.FragmentManager.executeOps(FragmentManager.java:2167)
        at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:1990)
        at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:1945)
        at androidx.fragment.app.FragmentManager.execSingleAction(FragmentManager.java:1816)
        at androidx.fragment.app.BackStackRecord.commitNow(BackStackRecord.java:297)
        at com.example.checklist.CreateTaskActivity$1.onItemSelected(CreateTaskActivity.java:53)
        at android.widget.AdapterView.fireOnSelected(AdapterView.java:1366)
        at android.widget.AdapterView.dispatchOnItemSelected(AdapterView.java:1355)
        at android.widget.AdapterView.access$300(AdapterView.java:59)
        at android.widget.AdapterView$SelectionNotifier.run(AdapterView.java:1314)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7078)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:494)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:974)

В чём заключается проблема?