androidstudioでの問題が解決しません

前提・実現したいこと

SQLiteでデータベースの読み書き処理を実装したいのですが、
エラーでアプリが落ちます

発生している問題・エラーメッセージ

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.example.innovation.f_featnote, PID: 2899
                  java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
                      at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:394)
                      at android.widget.ArrayAdapter.getView(ArrayAdapter.java:362)
                      at android.widget.AbsListView.obtainView(AbsListView.java:2344)
                      at android.widget.ListView.makeAndAddView(ListView.java:1864)
                      at android.widget.ListView.fillDown(ListView.java:698)
                      at android.widget.ListView.fillFromTop(ListView.java:759)
                      at android.widget.ListView.layoutChildren(ListView.java:1673)
                      at android.widget.AbsListView.onLayout(AbsListView.java:2148)
                      at android.view.View.layout(View.java:15596)
                      at android.view.ViewGroup.layout(ViewGroup.java:4966)
                      at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1703)
                      at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1557)
                      at android.widget.LinearLayout.onLayout(LinearLayout.java:1466)
                      at android.view.View.layout(View.java:15596)
                      at android.view.ViewGroup.layout(ViewGroup.java:4966)
                      at android.widget.FrameLayout.layoutChildren(FrameLayout.java:573)
                      at android.widget.FrameLayout.onLayout(FrameLayout.java:508)
                      at android.view.View.layout(View.java:15596)
                      at android.view.ViewGroup.layout(ViewGroup.java:4966)
                      at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1703)
                      at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1557)
                      at android.widget.LinearLayout.onLayout(LinearLayout.java:1466)
                      at android.view.View.layout(View.java:15596)
                      at android.view.ViewGroup.layout(ViewGroup.java:4966)
                      at android.widget.FrameLayout.layoutChildren(FrameLayout.java:573)
                      at android.widget.FrameLayout.onLayout(FrameLayout.java:508)
                      at android.view.View.layout(View.java:15596)
                      at android.view.ViewGroup.layout(ViewGroup.java:4966)
                      at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2072)
                      at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1829)
                      at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1054)
                      at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5779)
                      at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
                      at android.view.Choreographer.doCallbacks(Choreographer.java:580)
                      at android.view.Choreographer.doFrame(Choreographer.java:550)
                      at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
                      at android.os.Handler.handleCallback(Handler.java:739)
                      at android.os.Handler.dispatchMessage(Handler.java:95)
                      at android.os.Looper.loop(Looper.java:135)
                      at android.app.ActivityThread.main(ActivityThread.java:5221)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at java.lang.reflect.Method.invoke(Method.java:372)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

java

package com.example.innovation.f_featnote;


import android.app.Activity;

import android.app.ListActivity;

import android.content.Context;

import android.content.Intent;

import android.database.Cursor;

import android.database.sqlite.SQLiteDatabase;

import android.database.sqlite.SQLiteException;

import android.database.sqlite.SQLiteOpenHelper;

import android.os.Bundle;

import android.provider.ContactsContract;

import android.util.Log;

import android.view.LayoutInflater;

import android.view.View;

import android.view.ViewGroup;

import android.widget.AdapterView;

import android.widget.ArrayAdapter;

import android.widget.BaseAdapter;

import android.widget.Button;

import android.widget.ListView;

import android.widget.TextView;

import android.widget.Toast;


import com.example.innovation.f_featnote.SubOpenHelper;

import com.example.innovation.f_featnote.MainActivity;


import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;


public class SqlLoad extends Activity {

    ListView lv;

    static String[][] list;

    static String[] Did = new String[2];

    static long id_;

    static boolean[] flg;

    static String[] Data;

    private final int WC = ViewGroup.LayoutParams.WRAP_CONTENT;


    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.sql_load);

       // final boolean[] flg = new boolean[1];





        Main();
    }

    public void Main(){

        Data = SQLDataGet();
       // lv = new ListView(this);

    //    ArrayList<String> Data = new ArrayList<>(Arrays.asList(SQLDataGet()));

        lv = (ListView)findViewById(R.id.sql_load_main);

        ArrayAdapter<String> adapter = new ArrayAdapter<>(this, 
android.R.layout.simple_expandable_list_item_1, Data);

        lv.setAdapter(adapter);//リスト表示

     //   setContentView(lv,new ViewGroup.LayoutParams(WC,WC));


        lv.setOnItemClickListener(new AdapterView.OnItemClickListener(){
            @Override
            public void onItemClick(AdapterView<?> parent,View view,int position,long id){

                //  ListView listView = (ListView) parent;

                //   String item = (String)listView.getItemAtPosition(position);

                Log.d("id",""+id);

                id_ = id;
                SQLDContentGet();

                SQLDataSet();

                finish();

                // Toast.makeText(getApplicationContext(),item + "clicked",Toast.LENGTH_LONG).show();

            }

        });

    }





    public String[] SQLDataGet(){

        MainActivity main = new MainActivity();

        Context con = this;

        List ls = null;

        SQLiteDatabase sdb = main.SQLopen(con.getApplicationContext());//MainActivity参照

        final String[] columns = new String[]{"title","content","date"};//列名

        String where = null;//条件

        String[] Args = new String[]{""};//？の置き換え

        Cursor c = sdb.query("MemoContents", columns,where, null, null, null, null);

        int startPostion = c.getPosition();

        c.moveToFirst();

        list = new String[c.getCount()][columns.length];

        String[] Dlist = new String[c.getCount()];

        for (int i = 0; i< list[0].length-1; i++){

            for (int j = 0; j < list[1].length; j++){

                list[i][j] = c.getString(j);

                Log.d(i+"-"+j,list[i][j]);//デバッグ

                Dlist[i] = list[i][0];

            }

            c.moveToNext();

        }

        c.moveToPosition(startPostion);

      //  finish();

        return Dlist;
    }

    public void SQLDContentGet(){

        Did[0] = list[(int)id_][0];

        Did[1] = list[(int)id_][1];

    }

    public void SQLDataSet(){

        Intent data = new Intent();

        Bundle bundle = new Bundle();

        data.putExtra("key.data",Did);

        data.putExtras(bundle);


        setResult(RESULT_OK,data);

        finish();

    }

    @Override

    public void finish() {

        super.finish();

    }

}