During new site migration you may get below error as in picture with message like

"Uncaught Server Exception

There was an error building this page




Solution : 

For this you can run bench migrate and you will come and know which app you are required to switch branch 


For that go into the app folder and perform switch branch with example of steps below and check out  and after do bench migrate


cd apps/hrms 
git fetch upstream version-14:version-14 
git checkout version-14
 cd ../..


You can read more at : https://github.com/frappe/hrms/wiki/Changes-to-branching-and-versioning