QListWidget中的InternalMove使项目消失

I'm trying to make drag and drop inside a tree widget work to be able to move items around inside that widget. I've managed to make items draggable but when I release the mouse button the item disappears. To narrow it down I've tried the following example (taken from another post here on SO) which has the same issues as my tree widget in Qt 5.4 on Windows 7:I'm trying to make drag and drop inside a tree