fix: Fixed an issue where an exception would be thrown if the Campaig…

…n plugin was installed first, and then you newly installed SEOmatic ([#1530](#1530))

src/seoelements/SeoCampaign.php

@@ -442,6 +442,7 @@ public static function sourceHandleFromElement(ElementInterface $element)
     public static function createContentMetaBundle(Model $sourceModel)
     {
         /** @var CampaignTypeModel $sourceModel */
+        $sourceModel->attachBehavior('SEOmaticCampaignBehavior', CampaignBehavior::class);
         $sites = Craft::$app->getSites()->getAllSites();
         /** @var Site $site */
         foreach ($sites as $site) {