It seems like $channel can also be of type array; however, parameter $channel of Shieldon\Messenger\RocketChat::__construct() does only seem to accept string, maybe add an additional type check?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation
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return new RocketChat($accessToken, $userId, $serverUrl, /** @scrutinizer ignore-type */ $channel);