[core][dataset] Fix typo

darfix/core/dataset.py

@@ -721,8 +721,8 @@ class Dataset():
                 return
             new_data = Data(urls, self.running_data.metadata, self._in_memory)
 
-        if self.transformation:
-            transformation = [apply_2D_ROI(axis, origin, size, center) for axis in self.transformation]
+        transformation = ([apply_2D_ROI(axis, origin, size, center) for axis in self.transformation]
+                          if self.transformation else None)
         if indices is None:
             shape = list(self.data.shape)[:-2]
             shape.append(new_data.shape[-2])