Can't load the API viewer on /plugin/job-dsl/api-viewer/index.html with the following message:
Error while loading data, see Jenkins logs for details. Log is provided as an attachment
The class hudson.tools.JDKInstaller is missing. This has been removed from Jenkins core in 2.112 and is now provided by the JDK Tool plugin.
Check the plugin manager for missing plugins and install the JDK Tool plugin.
Thanks Daniel,
Installing the JDK plugin did fix my problem