Error when double clicking on the icon of 'drag & drop '
In the tree view when double clicking on the icon of 'drag & drop' the following error message appear:
File ".../tryton/tryton/gui/window/view_form/view/list.py", line 940, in __sig_switch
if column._type == 'button':
AttributeError: 'TreeViewColumn' object has no attribute '_type'