Add an assertion on the input image.

app.py

@@ -60,6 +60,8 @@ birefnet.eval()
 
 @spaces.GPU
 def predict(image, resolution, weights_file):
+    assert (image is not None), 'AssertionError: image cannot be None.'
+        
     if isinstance(image, str):
         response = requests.get(image)
         image_data = BytesIO(response.content)