ENGCOM-2031: [Backport] #14063 - Wrong invoice prefix in multistore s…

…etup due to default stor… #15821

 - Merge Pull Request #15821 from sanjay-wagento/magento2:2.1-develop-PR-port-15332
 - Merged commits:
   1. 2fcaf53

app/code/Magento/Sales/Model/ResourceModel/EntityAbstract.php

@@ -121,10 +121,15 @@ protected function _beforeSave(\Magento\Framework\Model\AbstractModel $object)
     {
         /** @var \Magento\Sales\Model\AbstractModel $object */
         if ($object instanceof EntityInterface && $object->getIncrementId() == null) {
+            $store = $object->getStore();
+            $storeId = $store->getId();
+            if ($storeId === null) {
+                $storeId = $store->getGroup()->getDefaultStoreId();
+            }
             $object->setIncrementId(
                 $this->sequenceManager->getSequence(
                     $object->getEntityType(),
-                    $object->getStore()->getGroup()->getDefaultStoreId()
+                    $storeId
                 )->getNextValue()
             );
         }