C＃：NullReferenceError - 实例化List <class>

Question

我试图创建一个保存数据库信息的类对象。不幸的是，它一直告诉我System.NullReferenceException并且我不明白为什么。

这是对象：

[Serializable]
public class DataBase
{
    public int NumberOfFilesBackedUp { set; get; } = 0;

    public class DataBaseItem // I tried struct too, but also no-go
    {
        public DateTime timeStamp { set; get; }
        public string src { set; get; }
        public string dst { set; get; }
        public int type { set; get; }
        public IntPtr pid { set; get; }
    }

    public List<DataBaseItem> BackupList { set; get; }

    public DataBase()
    {
        BackupList = new List<DataBaseItem>();
    }

}

在主应用程序（wpf）中：

public partial class App : Application
{
    public static DataBase DBObj; // declaring the object
    ...
    public App()
    {
    ...
        // Create/Load DB-Object
        CreateLoadDB();
    }


    private void CreateLoadDB()
    {
        ...
        try
        {
            DBObj = Pickler.ReadFromBinaryFile<DataBase>(DBObjFile);
            ...
        }
        catch (FileNotFoundException)
        {
            DBObj = new DataBase(); // instantiated, in debugging the code always comes here, as I don't yet have a serialized save file for any past objects

    }

    private void BackupManager_BackupEvent(string SrcPath, string DstPath, int Deleted, IntPtr Pid)
    {
       ...
        try
        {
            var DBItem = new DataBase.DataBaseItem(){ timeStamp = timestamp, src = SrcPath, dst = DstPath, type = Deleted, pid = Pid};
            DBObj.BackupList.Add(DBItem); // This throws the error... 

        }

DBobj未实例化。如果我在错误之前再次实例化它，那么没问题。但是我不想在那里实例化它，我需要在它当前被实例化的地方实例化它。

我不认为这是重复的，我知道Null Reference Exceptions是什么 - 我试图理解为什么我在这里得到它，因为我之前清楚地实例化了这个对象。