Fixing MediaWiki CategoryTree GetModuleStyles error

Since updating to version 1.26 of MediaWiki, category pages with CategoryTree subcategories would display the following error:

Warning: OutputPage::getModuleStyles: style module should define its position explicitly: ext.categoryTree.css ResourceLoaderFileModule [Called from OutputPage::getModuleStyles in /home/$USER/public_html/includes/OutputPage.php at line 623] in /home/$USER/public_html/includes/debug/MWDebug.php on line 300

Warning: OutputPage::getModuleStyles: style module should define its position explicitly: ext.categoryTree.css ResourceLoaderFileModule [Called from OutputPage::getModuleStyles in /home/$USER/public_html/includes/OutputPage.php at line 623] in /home/$USER/public_html/includes/debug/MWDebug.php on line 300

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