Fixed an AttributeError in DragStartEventListener.
Fixed an AttributeError in DragStartEventListener.

The 'buttonDownObj' member variable can already be None if another
drag/drop operation starts and completes  within the
'dragStartOperationRequested' signal propagation.

Use the local stack variable to access the button instead.

(transplanted from orange/872337b35d3c9c80650eacc5b2a6147446d17b01)

kwikadi/orange3,marinkaz/orange3,qusp/orange3,marinkaz/orange3,kwikadi/orange3,cheral/orange3,qPCR4vir/orange3,cheral/orange3,marinkaz/orange3,qusp/orange3,qPCR4vir/orange3,cheral/orange3,cheral/orange3,kwikadi/orange3,qusp/orange3,kwikadi/orange3,qPCR4vir/orange3,marinkaz/orange3,cheral/orange3,kwikadi/orange3,qPCR4vir/orange3,cheral/orange3,marinkaz/orange3,qPCR4vir/orange3,kwikadi/orange3,marinkaz/orange3,qPCR4vir/orange3,qusp/orange3