Update app.py

app.py

@@ -91,6 +91,9 @@ def transparent(fg, bg, alpha_factor):
     return background
 
 def show_img(all_imgs, dropdown, bg, alpha_factor):
+    if all_imgs is None: 
+        return None 
+        
     idx = target_list_all.index(dropdown)
     fg = all_imgs[idx]