Update main.py

main.py

@@ -595,7 +595,7 @@ class WatermelonProcessor:
         mark("render")
 
         return ProcessResult(
-            success=True, message="Success", r2_score=float(r2),
+            success=True, message="Success", r2_score=float(r2) if r2 is not None else None,
             width_val=width_val, height_val=height_val, perimeter_val=perimeter_val,
             total_area=total_area, flesh_area=flesh_area, flesh_area_ratio=flesh_area_ratio,
             elongation_factor=elongation_factor, circularity=circularity,