While updating my TYPO3 project from v7 to v10, I encountered some difficulties with my custom extensions. Due to that I just tried to rebuild the demo extension from the official TYPO3 website: https://docs.typo3.org/m/typo3/book-extbasefluid/10.4/en-us/4-FirstExtension/2-create-folder-structure-and-configuration-files.html to see what has changed over the years. I followed the instruction as stated on the website. After successfully activating the extension, I browsed to the