Steps to reproduce the issue

• Install Joomla! 4

• Select Extensions | Templates and select on the left Templates and click Cassiopeia Details and Files

• Click the toolbar button Copy Template and choose a name and click on Copy Template

Expected result

The template files are copied

Actual result

An error has occurred.
0 Class 'InstallerModelInstall' not found

You are forwarded to the url http://...joomla-cms/administrator/index.php?option=com_templates&task=template.copy&id=510&file=aG9tZQ
and you see in your browser what the following picture shows