How to fix com.apple.dock.extra?
You could try this fix:

In Finder use the Go to and go to /System/Library/CoreServices/Dock.app/Contents/XPCServices

In XPCServices Folder and move the com.apple.dock.extra.xpc file to desktop.

Then go back to the activity monitor and force quit the process and test!

Source: https://discussions.apple.com/thread/4263912 user jximo 07