This block of code assumes that the only place where a StorageUnavailableException can be thrown is in restoreState(), which re-sets the objects _yieldable pointer. If somewhere else were to throw, the invariant (now tassert) around _yieldable would trip.

While in practice we do not believe it is possible for any other code to throw StorageUnavailableException, we should nevertheless make the code safe in this situation.