C#

public sealed class Notifier
{
    static Notifier instance = null;
    static readonly object padlock = new object();
    System.Timers.Timer timer = new System.Timers.Timer(10000);
    static HashSet<UserInfo> users = new HashSet<UserInfo>();

    Notifier()
    {
       SetUpTimer();
    }

    public static Notifier Instance
    {
        get
        {
            lock (padlock)
            (
                if (instance == null)
                {
                   instance = new Notifier();
                }
                   return instance;
                }
            }
        }
    }
    
    public void ConnectUser(UserInfo user)
    {
        foreach (UserInfo info in users)
        {
            if ((info.UserName == user.UserName))
                return;
        }
        users.Add(user);
    }

    void timer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
    {
        if (users.Count == 0)
            return;
        timer.Enabled = false;
     
        //A lot of code...
   
        timer.Enabled = true;
    }
}

private void Callbutton_Click(object sender, RoutedEventArgs e) 
{  
 tab.AddPresetmenu("CALL BUTTON");         
}

ObservableCollection<DataItem> items = new ObservableCollection<DataItem>(); 
public  void AddPresetmenu(string pMenu) 
        {  
            items.Add(new DataItem(pMenu)); 
            menubox.ItemsSource = items; 
            menubox.UpdateLayout();
        }

<ListBox x:Name="menubox" ItemsSource='{Binding}' Margin="0,5,0,0" Background="Silver" BorderThickness="0">                <br />
<ListBox.ItemTemplate><DataTemplate><br />
TextBlock Foreground="Blue" FontWeight="Bold" Text="{Binding Path=Name}" TextWrapping="Wrap"<br />
VerticalAlignment="Center"/><br />
 </DataTemplate><br />
 </ListBox.ItemTemplate><br />
 </ListBox>

for(int i = 0; i<AnimalsDataGridView.Rows.Count; i++)
{
   try
   {
    if (File.Exists(AnimalsDataGridView.Rows[i].Cells[4].Value.ToString()))
    {
      //get the filePath
      String currentPath = AnimalsDataGridView.Rows[i].Cells[4].Value.ToString();
      Image img = Image.FromFile(currentPath);
                        
      Image thumbnail = SetThumbnail(img, 200, 200);

      //get the title of the image and store in imageTitle variable
                        
       //save image to byte array
       byte[] imageInfo = ImageToByteArray(thumbnail);
                        
       //Primary key of the database. I have not enabled the autogeneration feature regarding this.
        Guid guid = getGuid(thumbnail);

       this.ZooTableAdapter.Update(guid, imageTitle, imageInfo, currentPath, guid);
       this.ZooTableAdapter.Fill(ZooDataSet.Animals);
    }
                    
  }
  catch (Exception e)
  {
    Console.WriteLine(e.Message.ToString());
  }
}

if (File.Exists(AnimalsDataGridView.Rows[i].Cells[4].Value.ToString()))

NullReference Exception was unhandled
Object reference not set to an instance of an object.

UPDATE [Animals] SET [Id] = @p1, [Title] = @p2, [Thumbnail] = @p3, [FullPath] = @p4 WHERE (([Id] = @p5))