As Walter mentioned in the comments, you can try generating the preferences again by executing the command given below at MATLAB Command Window, or by renaming the existing preference directory.
One possible reason for this issue could be custom networking configuration of your computer. To workaround that, add the following line to the /private/etc/hosts file:
After making this change, execute the command:
dscacheutil -flushcache;sudo killall -HUP mDNSResponder
After doing this, restart (or start) MATLAB and try using App Designer again.
The following site has instructions for updating the hosts file: