因为异常'NSInvalidArgumentException'而取消拖动：尝试插入nil对象

Question

当我试图拖动＆amp;将对象从NSOutlineView拖放到另一个对象，通常可以正常工作，但有时对某些对象会失败。我收到了消息：

*** Canceling drag because exception 'NSInvalidArgumentException' (reason '*** -[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[0]') was raised during a dragging session

我发现当我第一次将项目拖到同一个表的另一行（不释放它）时会发生这种情况，然后拖到新表中。

这是我在拖动过程中可以检查的最后一个函数中draggedItems的内容：

- (id <NSPasteboardWriting>)outlineView:(NSOutlineView *)outlineView pasteboardWriterForItem:(id)item
{
    CBCollectible *collectible = [item representedObject];
    return [[collectible UniqueID] stringValue];
}

- (void)outlineView:(NSOutlineView *)outlineView
    draggingSession:(NSDraggingSession *)session
       endedAtPoint:(NSPoint)screenPoint
          operation:(NSDragOperation)operation
{
    //CBDebug(@"end draggingLeaderIndex %i", session.draggingLeaderIndex);
}

- (void)outlineView:(NSOutlineView *)outlineView
    draggingSession:(NSDraggingSession *)session
   willBeginAtPoint:(NSPoint)screenPoint
           forItems:(NSArray *)draggedItems
{
    FreeAndNil(theDraggedItems);
    theDraggedItems = [[NSArray alloc] initWithArray:draggedItems];
    [session.draggingPasteboard setData:[NSData data] forType:MAIN_VIEW_PASTEBOARD_TYPE];
}

- (void)outlineView:(NSOutlineView *)outlineView
updateDraggingItemsForDrag:(id<NSDraggingInfo>)draggingInfo
{
}

输出日志：

NSLog(@"draggedItems %@", draggedItems);
<__NSArrayM 0x31fa0f0>(
<NSTreeControllerTreeNode: 0x1d23750>, child nodes {}
)

更新：拖动注册

[outlineView1 registerForDraggedTypes:@[MAIN_VIEW_PASTEBOARD_TYPE, NSStringPboardType, NSFilenamesPboardType]];
[outlineView2 registerForDraggedTypes:@[MAIN_VIEW_PASTEBOARD_TYPE, NSStringPboardType, NSFilenamesPboardType]];