Xamarin - ExpandableListViewの親リストが画面遷移で消えてしまう

Xamarinでのアプリ開発でExpandableListViewを使用しているのですが、アプリ内で追加した親リストが画面遷移で消えてしまいます。
OnSaveInstanceStateを使えばいいのかなと思うのですが、使い方がいまいちわかりません。またほかに原因・対策が分かれば教えていただきたいです。

using Android.App;
using Android.Widget;
using Android.OS;
using System.Collections.Generic;
using Android.Content;
using PCLStorage;
using Android.Runtime;
using Temo1.Resources.layout;
using Android.Views;
using static Android.Resource;

namespace Temo1
{
    [Activity(Label = "Temo1", MainLauncher = true)]
    public class MainActivity : Activity
    {
        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);

            // Set our view from the "main" layout resource
            SetContentView(Resource.Layout.Main);



            Button memobutton = FindViewById<Button>(Resource.Id.button1);
            Button notebutton = FindViewById<Button>(Resource.Id.notebutton);
            ExpandableListView memolist = FindViewById<ExpandableListView>(Resource.Id.expandableListView1);

            string[] filename = new string[100];
            int n;
            var localFolder = FileSystem.Current.LocalStorage;
            var files = localFolder.GetFilesAsync().Result;


            //親リストgroupList子リストchildList
            JavaList<IDictionary<string, object>> groupList = new JavaList<IDictionary<string, object>>();
            JavaList<IList<IDictionary<string, object>>> childList = new JavaList<IList<IDictionary<string, object>>>();

            //デフォルトのグループ"全て"
            JavaDictionary<string, object> groupElement = new JavaDictionary<string, object>();
            groupElement.Add("GROUP_TITLE", "ALL");
            groupList.Add(groupElement);





                //子リスト用の文字列(ファイル名)を配列に用意
                n = 0;
                foreach (var file in files)
                {
                    var fn = file.Name;
                    fn = fn.Remove(fn.Length - 4);//拡張子を表示しないように末尾を削除
                    filename[n] = fn;//配列にファイル名を格納
                    n++;
                 }


            JavaList<IDictionary<string, object>> ChildElements = new JavaList<IDictionary<string, object>>();
            for (int j = 0; j <= n - 1; j++)
            {
                JavaDictionary<string, object> child = new JavaDictionary<string, object>();
                child.Add("CHILD_TITLE", filename[j]);
                ChildElements.Add(child);
            }
            childList.Add(ChildElements);

            SimpleExpandableListAdapter adapter = new SimpleExpandableListAdapter(
                                          this,
                                          groupList,
                                          Android.Resource.Layout.SimpleExpandableListItem1,
                                          new string[] { "GROUP_TITLE" },
                                          new int[] { Android.Resource.Id.Text1 },
                                          childList,
                                          Android.Resource.Layout.SimpleExpandableListItem2,
                                          new string[] { "CHILD_TITLE" },
                                          new int[] { Android.Resource.Id.Text2 }
                                          );
            memolist.SetAdapter(adapter);


            //親リスト追加

            //リスト名入力ダイアログ
            notebutton.Click += (_, __) =>
            {

                var layout = LayoutInflater.Inflate(Resource.Layout.dlg1, null);

                var dlg = new AlertDialog.Builder(this);
                dlg.SetTitle("ノート名");
                dlg.SetView(layout); //<=axmlで指定したビューを指定
                dlg.SetPositiveButton("OK", (s, a) => {
                    //axml内のビューへのポインタの取得
                    var notename = layout.FindViewById<EditText>(Resource.Id.dlgtxt);
                    Toast.MakeText(this, notename.Text, ToastLength.Short).Show();

                    if (!string.IsNullOrEmpty(notename.Text))
                    {
                        var newGroupElement = new JavaDictionary<string, object>();
                        newGroupElement.Add("GROUP_TITLE", notename.Text);
                        groupList.Add(newGroupElement);

                        childList.Add(new JavaList<IDictionary<string, object>>());
                        adapter.NotifyDataSetChanged();
                    }
                    else
                    {
                        //　未入力エラー
                        Toast.MakeText(this, "未入力です", ToastLength.Short).Show();
                    }
                });
                dlg.Create().Show();


                };


            //メモ作成画面へ遷移
            memobutton.Click += (_, __) =>

            {

                var intent = new Intent(this, typeof(memocreate));

                StartActivity(intent);


            };


            //リストアイテムがクリックされたらテキスト編集に遷移
            memolist.ChildClick+= async (sender, e) =>
            {
                var parent = (ExpandableListView)e.Parent;
                var packedPosition = parent.GetExpandableListPosition(e.ChildPosition);

                var groupPosition = ExpandableListView.GetPackedPositionGroup(packedPosition);
                var childPosition = ExpandableListView.GetPackedPositionChild(packedPosition);
                var type = ExpandableListView.GetPackedPositionType(packedPosition);
                if (type == PackedPositionType.Child)
                {
                    var item = (IDictionary<string, object>)parent.ExpandableListAdapter.GetChild(groupPosition, childPosition+1);
                    var clickfile = item["CHILD_TITLE"].ToString();

                    //そのファイルのテキストを取得
                    IFolder rootFolder = FileSystem.Current.LocalStorage;
                    IFile file = await rootFolder.GetFileAsync(clickfile + ".txt");//ファイルを読むために拡張子をつける
                    string saveddata = await file.ReadAllTextAsync();

                    var intent = new Intent(this, typeof(memoedit));
                    intent.PutExtra("clickfile", clickfile); // intentに値を入れる
                    intent.PutExtra("Data", saveddata);


                    StartActivity(intent);

                }
            };



                //長押しされたアイテムを削除
                memolist.ItemLongClick += async (sender, e) =>
            {
                var parent = (ExpandableListView)e.Parent;
                var packedPosition = parent.GetExpandableListPosition(e.Position);

                var groupPosition = ExpandableListView.GetPackedPositionGroup(packedPosition);
                var childPosition = ExpandableListView.GetPackedPositionChild(packedPosition);
                var type = ExpandableListView.GetPackedPositionType(packedPosition);
                if (type == PackedPositionType.Child)
                {
                    var item = (IDictionary<string, object>)parent.ExpandableListAdapter.GetChild(groupPosition, childPosition);
                    var deletefile = item["CHILD_TITLE"].ToString();


                    //adapterから削除し更新
                    childList[groupPosition].Remove(item);
                    adapter.NotifyDataSetChanged();

                    //元ファイル削除
                    IFolder rootFolder = FileSystem.Current.LocalStorage;
                    IFile file = await rootFolder.GetFileAsync(deletefile + ".txt");

                    await file.DeleteAsync();
                }


            };


            //検索

            EditText searchtext = FindViewById<EditText>(Resource.Id.searchtext);

            searchtext.Click += (sender, e) =>
            {
                searchtext.Text = "";

            };







        }
    }
}