update to Arduino 1.6.6 gives WARNING: Error loading hardware folder

Development Tools IDE 1.x
1

Any idea how to fix this (warning given by compiler after upgrade to Arduino 1.6.6)?:

WARNING: Error loading hardware folder C:\...\Documents\Arduino\hardware\breadboard
  No valid hardware definitions found in folder breadboard.

I'm using a Gemma v2 as my hardware, and I do have the latest libraries. This error/warning did not occur in Arduino 1.6.4

2

You have a folder called breadboard inside your hardware folder, but it's not a valid core. Maybe an old core from when you were using 1.0.x? Older 1.6.x versions did not warn about this