error - add list items to the static class variable C#

Question

I want to get list listview items in the static class. Because I want to pass this class variable to another class. So I have tried... 

public static class FileMngUserID
       {
           public static List<int> UserId;
           public static List<string> ImgName;
       }

foreach (DocumentsUser item in listView1.SelectedItems)
           {
               FileMngUserID.UserId.Add(Convert.ToInt32(item.UserId));
               FileMngUserID.ImgName.Add(item.Parent_File_Name);
           }

 but here  FileMngUserID.UserId.Add(Convert.ToInt32(item.UserId)); occurring error 'Object reference not set to an instance of an object.'