Fatal error: Cannot declare class - Joomla 3.7

That correct the issue, but there is a new issue in the backend if your component has "categories" view. As the CategoriesHelper (categories.php) of com_categories search for your component helper named "example.php" for render the sidebar and now we have to renamed to "exampleadmin.php" then it is not founded so the administration categories sidebar is empty!!!

As we can read in the section "Adding categories" of the Joomla3 tutorial "J3.x:Developing an MVC Component"

[link]docs.joomla.org/J3.x:Developing_an_MVC_C...nt/Adding_categories[/link]

Could you team please fix this issue?!?!?

Best regards.

Carlos

As described where? My backend no longer functions either, and I'm not exactly sure what I'm adding or to which files.