WPF MVVM 패턴을 이용한 navigation시 AssociatedObject null 체크

[redstone]

안녕하세요. 항상 도움을 많이 받고 있습니다
frame을 이용하여 버튼 클릭시 navigation되는 로직을 짜고 있습니다.
초기값을 설정하지 않고 그냥 네비게이션 할때는 괜찮은데 뷰모델에서 프로그램 실행시에 최초로 보여질 화면을 초기값으로 주면
System.NullReferenceException: ‘Object reference not set to an instance of an object.’ 에러를 뱉습니다

Behavior.cs

using Microsoft.Xaml.Behaviors;
using System;
using System.Windows;
using System.Windows.Controls;


   public class FrameBehavior : Behavior<Frame>
    {
        private bool _isWork;

        protected override void OnAttached()
        {
            
            AssociatedObject.Navigating += AssociatedObject_Navigating;
            
            AssociatedObject.Navigated += AssociatedObject_Navigated;
        }
       
        private void AssociatedObject_Navigated(object sender, System.Windows.Navigation.NavigationEventArgs e)
        {
            _isWork = true;
            
            NavigationSource = e.Uri.ToString();
            _isWork = false;
          
            if (AssociatedObject.Content is Page pageContent
                && pageContent.DataContext is INavigationAware navigationAware)
            {
                navigationAware.OnNavigated(sender, e);
            }
        }
      
        private void AssociatedObject_Navigating(object sender, System.Windows.Navigation.NavigatingCancelEventArgs e)
        {
            //네비게이션 시작전 상황을 뷰모델에 알려주기
            if (AssociatedObject.Content is Page pageContent
                && pageContent.DataContext is INavigationAware navigationAware)
            {
                navigationAware?.OnNavigating(sender, e);
            }
        }

        protected override void OnDetaching()
        {
            AssociatedObject.Navigating -= AssociatedObject_Navigating;
            AssociatedObject.Navigated -= AssociatedObject_Navigated;
        }

        public string NavigationSource
        {
            get { return (string)GetValue(NavigationSourceProperty); }
            set { SetValue(NavigationSourceProperty, value); }
        }

        /// <summary>
        /// NavigationSource DP
        /// </summary>
        public static readonly DependencyProperty NavigationSourceProperty =
            DependencyProperty.Register(nameof(NavigationSource), typeof(string), typeof(FrameBehavior), new PropertyMetadata(null, NavigationSourceChanged));

        
        private static void NavigationSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            var behavior = (FrameBehavior)d;
            if (behavior._isWork)
            {
                return;
            }
            behavior.Navigate();
        }
        /// <summary>
        /// 네비게이트
        /// </summary>
        private void Navigate()
        {
            switch (NavigationSource)
            {
            
                default:
                    //navigate
                    AssociatedObject.Navigate(new Uri(NavigationSource, UriKind.RelativeOrAbsolute));
                    break;
            }
        }
    }
}

MainWindowViewModel

  private void init()
        {
            NavigationSource = "View/Dashboard.xaml"; //실행되자마자 로드되어야할 뷰
            NavigateCommand = new RelayCommand<string>(OnNavigate);
        }
 private void OnNavigate(string pageUri)
        {
            NavigationSource = pageUri;
        }

 private string _navaigationSource;

       public string NavigationSource
        {
            get => _navaigationSource;
            set => SetProperty(ref _navaigationSource, value);
        }

public ICommand NavigateCommand { get; set; }

 public MainWindowViewModel()
        {
            
            init();
        }

``초기값을 주지않고 그냥 navigation할때```