fix: do not attempt to remove non-existing image grid file (#2456)

image grid is actually not an image here but a numpy array, as the grid isn't saved by default

webui.py

@@ -76,6 +76,7 @@ def generate_clicked(task):
                 # delete Fooocus temp images, only keep gradio temp images
                 if args_manager.args.disable_image_log:
                     for filepath in product:
+                        if isinstance(filepath, str) and os.path.exists(filepath):
                             os.remove(filepath)
 
     execution_time = time.perf_counter() - execution_start_time