Xamarin Forms overriding OnAppearing causes app to crash

I am trying to do a simple override and load some data when my page loads, I am using the following code in the code behind page.

namespace XYZ
{
    public partial class MainPage : ContentPage
    {
        private Label results;
        private Label groupResults;

        public MainPage()
        {
            InitializeComponent();

            results = new Label();

            groupResults = new Label();
        }

        protected override void OnAppearing()
        {
            base.OnAppearing();
            storeIdTxt.Text = Settings.StoreIdSetting;
        }
}
}

If I uncomment the override things works just fine, the error I am getting seems to be a generic one attached here

my settings class is fairly simple as follows

using System;
using System.Collections.Generic;
using System.Text;
using Plugin.Settings;
using Plugin.Settings.Abstractions;

namespace NWMPosNG.Helpers
{
  /// <summary>
  /// This is the Settings static class that can be used in your Core solution or in any
  /// of your client applications. All settings are laid out the same exact way with getters
  /// and setters. 
  /// </summary>
  public static class Settings
{
    private static ISettings AppSettings
    {
        get
        {
            return CrossSettings.Current;
        }
    }

    #region Setting Constants

    private const string SettingsKey = "settings_key";
    private static readonly string SettingsDefault = string.Empty;

    private const string StoreId = null;
    private static readonly string StoreIdDefault = "0";

    #endregion


    public static string GeneralSettings
        {
            get
                {
                    return AppSettings.GetValueOrDefault(SettingsKey, SettingsDefault);
                }

            set
                {
                    AppSettings.AddOrUpdateValue(SettingsKey, value);
                }
        }

    public static string StoreIdSetting
        {
            get
            {
                return AppSettings.GetValueOrDefault(StoreId, StoreIdDefault);
            }

            set
            {
                AppSettings.AddOrUpdateValue(StoreId, value);
            }
        }
}
}

I narrowed down the issue to when I access the saved data using

storeIdTxt.Text = Settings.StoreIdSetting;

But I don't understand why that causes the crash.