Fatal error: Call to undefined function enableSemantics()

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Administrator manualInstallationInstallation/TroubleshootingFatal error: Call to undefined function enableSemantics()
Fatal error: Call to undefined function enableSemantics() in ... LocalSettings.php
Something went wrong with the registration or installation, please re-read the instillation instructions. 1
Fatal error: Call to undefined function enableSemantics()
This kind of message appears if the Extension "Validator" extension is still explicitly invoked. To resolve this the following line has to be removed:
require_once "$IP/extensions/Validator/Validator.php";
Fatal: Call to undefined function enableSemantics() …
This kind of message appears for MW ≤ 1.21.x if the Extension Installer extension is included after the call to enableSemantics() (see above). Make sure that the call to include Extension Installer comes first.