Unity3d: No Drag&Drop has been setup

Question

I was working on a Unity 2D game project on OS X 10.10, and I can't drag file or folder, it will show this error.

No Drag&Drop has been setup. Please
UnityEditor.DockArea:OnGUI()

I have tried:

1. save project and then restart
2. click Assets folder and choose Reimport All

But it seems not work, how to fix it?

Answer 1

Recently I have the same problem and what I did was quit Unity and force quit the pboard process via Activity Monitor and then launch Unity again, it works for me.