Why is my Launchpad missing app icons?
This didn't work for me,
Finder did not respond, clicking on Apps gave this error  

Could not obtain valid temporary directory for sandboxed app ...


I also had to open a terminal and run:

cd /var/folder
sudo rm -rf *


Searched for and opened Activity Monitor, stopped the com.apple.IconServicesAgent service and rebooted.

My icons came back in launchpad and dock.