Thanks for the information. I checked the Farming System plugin and it does not contain or use a property named _rngSeed, so the error may be coming from another plugin or a plugin compatibility/order issue.
Could you please press F8 when the error appears, open the Console tab, and send me a screenshot of the full red error stack, including the plugin filenames and line numbers?
Please also let me know which other plugins are enabled and whether more than one version of CT Farming System is present in Plugin Manager.