CHUI-881 ([CHUIBUG]Viewer crashes when user drag and drop the participant from...

CHUI-881 ([CHUIBUG]Viewer crashes when user drag and drop the participant from the people floater to the one of the tool bars): Problem was that the item dragged from the people list was not of type inventoryitem. Instead of casting to an inventoryitem and checking if the item being dragged is a widgit, instead look at the cargo type (which also specifies that the dragged item is a widgit but doesn't need to typecast).