Since the new Widgets Explorer was added to Plasma, the Add Widgets… options from the Panel context menu was not working. I thought it might be fixed later because Widgets Explorer was in heavy development.

Later, on careful observation I found out that the signal is connected to PanelController, which is only shown when the panel toolbox is open.
I submitted my solution which made the PanelView class handle the signal if the toolbox is not open and after some modifications to other libraries by Aaron, the fix was commited.