[diffusion] fix: webui cannot correctly display generated video using wan2.2 (#18473)

Signed-off-by: Xiaodong Ye <xiaodong.ye@mthreads.com>
Co-authored-by: Mick <mickjagger19@icloud.com>

python/sglang/multimodal_gen/apps/webui/main.py

@@ -111,9 +111,17 @@ def run_sgl_diffusion_webui(server_args: ServerArgs):
             sampling_params_str = "\n".join(
                 [f"{key}: {value}" for key, value in sampling_params_kwargs.items()]
             )
-            raise ValueError(
-                f"No output is generated by client, and their sampling params is: {sampling_params_str}"
-            )
+            no_output_msg = f"No output is generated by client, and their sampling params is: {sampling_params_str}"
+
+            if batch.data_type == DataType.VIDEO:
+                if os.path.exists(save_file_path):
+                    logger.warning(no_output_msg)
+                    return None, save_file_path
+                else:
+                    no_output_msg += f"\nAnd the expected output file was not found at: {save_file_path}"
+                    raise ValueError(no_output_msg)
+            else:
+                raise ValueError(no_output_msg)
 
         frames = post_process_sample(
             result.output[0],