After release 1.5.0 sellacious giving trouble with Joomla 3.8, it may be Joomla issue, Abhishek Sir, do not recommend Joomla 3.8 update to users, please send an email everyone, do not update Joomla without test it on the testing server.
I am not blaming sellacious and/or Joomla, used Template, plugins and modules may be not supported to Joomla 3.8
For all e-commerce operator. The best solution is to test it on the testing server and resolve the issues then update the Live Site.