CamBam 0.9.8 assumes there is a top level style with a blank name.
If you delete or rename this style, you will get the error you are seeing. The easiest way to fix this is to create a new style and delete the name.
This will then become your default style. All other styles in that library will ultimately refer to this one. Also if you create a drawing and insert a machining operation without specifying any style anywhere, the default style will be used to resolve machining operation properties marked as 'default'.
It is a good idea to load the default style up with all your common settings such as clearance plane, general purpose speeds and feeds etc.
You should then find, when you create new styles you may just need to adjust one or two values away from the default.
I am just doing some final testing on a new 0.9.8F release which should make maintaining the style libraries a little easier. If nothing major crops up that should be available in the next couple of days.