Fix ROI metadata handling: ensure backward compatibility

Author: PierreRaybaut

cdl/core/model/base.py

@@ -1463,9 +1463,12 @@ def roi(self) -> TypeROI | None:
             Regions of interest object
         """
         roidata = self.metadata.get(ROI_KEY)
-        assert roidata is None or isinstance(roidata, dict)
         if roidata is None:
             return None
+        if not isinstance(roidata, dict):
+            # Old or unsupported format: remove it
+            self.metadata.pop(ROI_KEY)
+            return None
         return self.get_roi_class().from_dict(roidata)
 
     @roi.setter