How to fix "com.apple.dock.extra: ... Could not connect the action buttonPressed: to target of class NSApplication" in system.log
You have a similar request to this question:

How to fix message spam from usbmuxd in console (_SendDetachNotification/_SendAttachNotification)

The answer I put there should work if this hasn't fixed itself via time or an upgrade to a newer OS. Just send the com.apple.dock.extra messages to /dev/null by editing /etc/syslog.conf