[grammar] Avoid server crash when grammar backend is None (#11401)

python/sglang/srt/managers/scheduler.py

@@ -1442,7 +1442,10 @@ class Scheduler(
             or req.sampling_params.ebnf is not None
             or req.sampling_params.structural_tag is not None
         ):
-            assert self.grammar_backend is not None
+            if self.grammar_backend is None:
+                error_msg = "Grammar-based generation (json_schema, regex, ebnf, structural_tag) is not supported when the server is launched with --grammar-backend none"
+                req.set_finish_with_abort(error_msg)
+            else:
                 if req.sampling_params.json_schema is not None:
                     key = ("json", req.sampling_params.json_schema)
                 elif req.sampling_params.regex is not None: