Trouble upgrading CiviCRM 5.12.4 to 5.13.5 with Sparkpost extension
I'm unsure whether this is an issue with your extension. Please see this post: https://civicrm.stackexchange.com/questions/30975/upgrade-from-civicrm-5-12-4-to-5-13-5-generates-php-fatal-error
I'm attaching the error log that shows the error and the stack trace. I'm looking for any advice to get over this hump. error.log