If you don't have many other extensions installed, the best way to remove those errors would be to rename the configuration folder and install the rest of the products again.
The reason this error occurs on startup is that you probably had the floaters open in Dreamweaver (or when you shut down Dreamweaver) when you uninstalled Form Builder. Extension manager incorrectly uninstalls the files for them even though they're marked as shared.
Edit: After discussing with a coworker, a better way to resolve this issue is to go into Dreamweaver and go to the Window->Workspace Layout->Reset '<current workspace name>' menu. Then follow the common installation issues document and delete the winfilecache:
Sorry for the inconvenience this has caused. If this doesn't work, you would want to rename the configuration folder.