fix exception

pkg/thread.py

@@ -2,7 +2,6 @@ import shutil
 from pathlib import Path
 
 import requests
-from PIL import Image
 from tqdm import tqdm
 
 from pkg.proxies import PROXIES
@@ -23,7 +22,7 @@ def download_tile(task):
         output_path: Path = output / f"{row}_{col}.png"
         if output_path.exists():
             valid_png_file, image_type = is_png(output_path)
-            if convert_to_png and image_type != 'PNG':
+            if convert_to_png and image_type and image_type != 'PNG':
                 # The image was read sucessfully by PIL but it's in the wrong format.
                 convert(output_path)
                 if not is_png(output_path):
@@ -59,6 +58,7 @@ def download_tile(task):
             print(f"Failed to download tile {row}_{col}")
             return row, col, 'failure'
     except Exception as e:
+        # import traceback
         # traceback.print_exc()
         tqdm.write(f'Exception on {(row, col)} - {e.__class__.__name__}: {e}')
         return row, col, 'failure'