fixed a bug in pipeline selector

cdplib/pipeline_selector/PipelineSelector.py

@@ -189,7 +189,7 @@ class PipelineSelector(ABC):
             self.finished_tuning = False
             
             self._additional_metrics = additional_metrics
-            self._additional_averaging_funcs = additional_averaging_funcs
+            self._additional_averaging_funcs = additional_averaging_funcs or {}
 
         except Exception as e:
             err = ("Failed to initialize the class. "