Thank you for giving the steps to correct this problem. HOWEVER, for me it shows that GiveWP could do better with its programming guidance. This error could have been caught in code and instead of a website wide PHP fatal error, your code could have looked for the condition caused by Form Field Manager and instead given a softer notice, one that warned of the need to update that plugin; or, not updating the main Give plugin unless the Form Field Manager plugin was updated first; or, …(your developers should be able to come up with a better outcome than what you have now).
Crashing a website because your coding standards don’t catch a fatal error and deal with it effectively is a red flag.
I hope you can change your programming policy/guidelines to avoid this in future releases.