Launchpad Search Field not working - cannot type
Hit the tab key a few times. Worked for me.

You need to hit Tab once the launchpad screen comes up. The tab will then put focus on the search bar, and then pressing it again will remove it. Pressing it ~20times seems to cause some sort of reset. Once you visibly notice the cursor, things should be good. 

Other things to try: 


terminal > defaults write com.apple.dock ResetLaunchPad -bool true; killall Dock
logout / login
restart