Reset dock and rebuild stock icons as follows
Open finder window
HOLD Down the Alt (Option) key on KEYBOARD and simultaneously mouse to the Go Menu .
Continue to hold the Alt (Option) key and click on Library in the list (Release the Alt key the library item is hidden !)
Your finder window now shows the connect of the library folding in your home folder.
Set view to “as list” from menu bar
Scroll down to the preferences folder and click to open.
Locate file name : com.apple.dock.plist
Drag to trash .
Re boot Mac and above file will be recreated with all the default icons back in the dock.