In case you missed it, there is a problem in the Domino Designer after installing Feature Pack 8. Below the description of IBM Support.
An intermittent problem has been introduced in Domino Designer 9.0.1 Feature Pack 8 that causes a failure when opening the SSJS editor. This problem affects only applications that contain a managed bean.
This issue is being tracked as SPR LHEYAKBJSQ. As a workaround, users can perform a full clean/rebuild of the application and then use the SSJS editor. Note: The problem may reoccur when the application is closed and then reopened.
For more information : IBM Support
On the short term there will be a fix for this issue as indicated by Martin Donnelly
as a response to my tweet earlier today.