Merge pull request #5428 from eukaryo/gp-indep-multi-error

Check whether the study is multi-objective in `sample_independent` of `GPSampler`

optuna/samplers/_gp/sampler.py

@@ -206,6 +206,7 @@ def sample_independent(
         param_name: str,
         param_distribution: BaseDistribution,
     ) -> Any:
+        self._raise_error_if_multi_objective(study)
         return self._independent_sampler.sample_independent(
             study, trial, param_name, param_distribution
         )