fix in ImageWrapper destructor

nabirds/dataset/image.py

@@ -34,7 +34,8 @@ class ImageWrapper(object):
 		self._feature = None
 
 	def __del__(self):
-		self._im.close()
+		if isinstance(self._im, Image.Image):
+			self._im.close()
 
 	@property
 	def im(self):