GIT2-3613 fixed https://jira.sakaiproject.org/browse/SAK-26637 in the GB tool because this was easier than trying to fix it in the service.
GradebookNgBusinessService->getGradebook has the same problem here and in the case of the GradebookNotFoundException should just addGradebook.
You can use the same test as on that SAK just go to a DAC site (site/DAC-EDUCATION-DEPT1-SUBJ1-101) or something that was created by manually adding GB and it says
org.apache.wicket.WicketRuntimeException: Can't instantiate page using constructor 'public org.sakaiproject.gradebookng.tool.pages.GradebookPage()'. An exception has been thrown during construction!