Hi,
If you test prestashop version you are using on default theme and try adding products or any other operations. If there is no error and error comes only when new theme used.
Then it is clear problem is not from prestashop version but compatibility issues or any other problem with new theme.
PrestaShop Tutorials Videos [How to do Tasks]
https://www.prestashop.com/forums/topic/907438-prestashop-tutorials-videos-how-to-do-tasks/