[core][dataset] Use flattened array of data when obtaining metadata

darfix/core/dataset.py

@@ -406,7 +406,7 @@ class Dataset():
 
     def get_metadata_values(self, kind, key):
         return numpy.unique([data.get_value(kind=kind, name=key)[0] for data
-                             in self.data.metadata])
+                             in self.get_data().metadata])
 
     def __compute_changing_value(self, values, changing_value=1):
         """