[Maskformer] safely get backbone config (#29166)

Safe getattr

src/transformers/models/maskformer/modeling_maskformer.py

@@ -1439,7 +1439,7 @@ def __init__(self, config: MaskFormerConfig):
                 The configuration used to instantiate this model.
         """
         super().__init__()
-        if hasattr(config, "backbone_config") and config.backbone_config.model_type == "swin":
+        if getattr(config, "backbone_config") is not None and config.backbone_config.model_type == "swin":
             # for backwards compatibility
             backbone_config = config.backbone_config
             backbone_config = MaskFormerSwinConfig.from_dict(backbone_config.to_dict())