The property EnableSidecart does not exist on Dynamic\FoxyStripe\Model\FoxyStripeSetting. Since you implemented __get, consider adding a @property annotation.
Are you sure Dynamic\FoxyStripe\Model...:getFoxyCartStoreName() of type false|SilverStripe\ORM\FieldType\DBVarchar can be used in concatenation?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation