Add input validation to prevent None image errors

- Check if image is None before processing
- Validate image dimensions (must be 3D array)
- Return None, None gracefully if validation fails
- Fixes cv2.cvtColor() assertion error when no image uploaded

app.py

@@ -10,10 +10,20 @@ def detect_traffic_signs(image, confidence_threshold):
     """
     Process the uploaded image and return the image with detected signs.
     """
+    # Validate input image
+    if image is None:
+        print("No image provided")
+        return None, None
+    
     print(f"Received image type: {type(image)}")
     if hasattr(image, 'convert'):
         image = np.array(image)
         print(f"Converted PIL to numpy array, shape: {image.shape}")
+    
+    # Check if image is valid
+    if image.size == 0 or len(image.shape) != 3:
+        print(f"Invalid image: shape={image.shape}")
+        return None, None
 
     # Convert RGB to BGR for OpenCV
     image = cv2.cvtColor(image, cv2.COLOR_RGB2BGR)