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The error is due to the fact that neither g0 nor g1 are initialized. They have simply been declared as a metric and a type (0,2) tensor field, but you should initialize their components in some vector frame, in order to fully define them.

Regarding perturbation of tensor fields, note that tensor series expansion have been introduced in Sage 8.8, see the changelog for details, in particular cell [25] of this notebook for a concrete example of use.