TYPO3 GeneralUtility Class 0; not found (own generic extension)

After creating my own Extension with the ExtensionBuilder-Package and activating it, I created a repository for the data and set the Storage-PID in the Template-Constants.

Then I created a new page containing the extension (with default values) and on viewing the page the following error appears and the page does not show:

Core: Exception handler (WEB): Uncaught TYPO3 Exception: Class '0;' not found | Error thrown in file /var/www/html/typo3Insy/public/typo3/sysext/core/Classes/Utility/GeneralUtility.php

For the setup I used the TYPO3 version 9.5.23 and have not migrated from a previous one.

Solution

Thanks to @Jack70 I found out, that an autoloader needs to be supplied for my own extension in the composer.json-File of the TYPO3-installation itself.

From the composer.json-File of the created extension itself, you can extract the vendor name and the site package. Make sure the case matches when supplying it to the autoloader-section:

"autoload": {
        "psr-4": {
            "MyVendor\\MySitePackage\\": "pathToTheClassesDirectoryOfYourExtension"
        }
    }

After that you can simply run composer dump-autoload and this error will be fixed (without restarting the webserver).

I found out about this here: https://wiki.typo3.org/Exception/CMS/1289386765