Users usually encounter this situation editing a survey that they have been editing without problems before. However after making changes to the database, they found that
gfsurveys.DeleteQuestion: execution of BeforeUpdate caused by: System.NullPointerException: Attempt to de-reference a null object (gfsurveys)
Class.gfsurveys.SurveyFieldMappingController.obtainListOfSurveyMappingsWithRequiredRelations: line 963, column 1 Trigger.gfsurveys.DeleteQuestion: line 34, column 1
The most likely cause is that you or your Salesforce Administrators has deleted or renamed some fields and that has invalidated some questions.
Please see the article on Actions to Avoid while using TaroWorks for more information.
Either undo the changes to the database, or if the changes were valid, unmap the survey questions first, then edit the question and redo the field mapping.