validate optuna settings to ensure keys are dictionaries

Author: SvenKlaassen

doubleml/double_ml.py

@@ -1196,6 +1196,10 @@ def _validate_optuna_setting_keys(self, optuna_settings):
                 + "."
             )
 
+        for key in allowed_learner_keys:
+            if key in optuna_settings and not isinstance(optuna_settings[key], dict):
+                raise TypeError(f"Optuna settings for '{key}' must be a dict.")
+
     def _validate_optuna_param_space(self, ml_param_space):
         """Validate learner keys provided in the Optuna parameter space dictionary."""