Just a follow-up here: Deleting the cache file didn't stop the error for me either, but I went back and read the "sticky" post from Jason about v. 1.0.5 issues ("Design Extender 1.0.5 has been released") and tried the last trick he lists:
Originally Said By: Jason Byrnes
this can be corrected by resetting the workspace layout:
Window -> Workspace Layout -> Reset 'your workspace'
Actually, I couldn't resist taking a few extra steps, and maybe they are inconsequential, but I'll report: I switched back and forth to a few of the preconfigured workspaces, reset once again, deleted the newly-created cache file, then relaunched.
No error message!