Just purchased Premium and installing for the first time….
[31-Mar-2020 03:01:18 UTC] PHP Fatal error: method_exists(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "GeneratePress_Sites_Process_Beaver_Builder" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide an autoloader to load the class definition in /home/traded7/public_html/2020/wp-content/plugins/astra-sites/inc/importers/batch-processing/helpers/class-wp-background-process-astra.php on line 40
NOTE: I already had Beaver Builder Pro installed prior to GP Premium