Steps to reproduce

1. Upgrade from an old version to 6.2.7-SNAPSHOT
2. Create a new page and select Basic template
3. Restart the instance (optional?)
4.  Change the page template to Travel Home
5. Open the page

Expected results

Can open the new page

Actual results

The page's displayed error 500

Workaround

Delete the page and recreate and choose Travel Home from the beginning.

Development notes

Seems to happen on dx-core only, two people confirmed the issue, one could not reproduce it

Looking closer at the exception stack trace it seems that we should fix our template at https://git.magnolia-cms.com/projects/MODULES/repos/demo-projects/browse/enterprise/magnolia-travel-demo-marketing-tags/src/main/resources/travel-demo-marketing-tags/templates/pages/areas/htmlHeader-marketing-tags.ftl#21 by handling a possible null value?

See also Slack conversation https://magnolia-cms.slack.com/archives/CDF00BHMH/p1617029749034800?thread_ts=1616994005.033700&cid=CDF00BHMH