fix generate error (#1590)

### What problem does this PR solve?

#1550 #1210 

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

api/apps/canvas_app.py

     except Exception as e:
         return server_error_response(e)
 
-    assert answer, "Nothing. Is it over?"
+    assert answer is not None, "Nothing. Is it over?"
 
     if stream:
-        assert isinstance(answer, partial)
+        assert isinstance(answer, partial), "Nothing. Is it over?"
 
         def sse():
             nonlocal answer, cvs
         resp.headers.add_header("Content-Type", "text/event-stream; charset=utf-8")
         return resp
 
+    final_ans["content"] = "\n".join(answer["content"]) if "content" in answer else ""
     canvas.messages.append({"role": "assistant", "content": final_ans["content"]})
     if final_ans.get("reference"):
         canvas.reference.append(final_ans["reference"])
     cvs.dsl = json.loads(str(canvas))
     UserCanvasService.update_by_id(req["id"], cvs.to_dict())
-    return get_json_result(data=req["dsl"])
+    return get_json_result(data={"answer": final_ans["content"], "reference": final_ans.get("reference", [])})
 
 
 @manager.route('/reset', methods=['POST'])

deepdoc/parser/ppt_parser.py

                 break
             texts = []
             for shape in sorted(
-                    slide.shapes, key=lambda x: (x.top // 10, x.left)):
+                    slide.shapes, key=lambda x: ((x.top if x.top is not None else 0) // 10, x.left)):
                 txt = self.__extract(shape)
                 if txt:
                     texts.append(txt)

graph/component/arxiv.py

 
         df = pd.DataFrame(arxiv_res)
         if DEBUG: print(df, ":::::::::::::::::::::::::::::::::")
-        return df
+        return df

graph/component/generate.py

             # prompt = re.sub(r"\{%s\}"%n, re.escape(str(v)), prompt)
             prompt = re.sub(r"\{%s\}" % n, str(v), prompt)
 
-        if kwargs.get("stream"):
+        downstreams = self._canvas.get_component(self._id)["downstream"]
+        if kwargs.get("stream") \
+                and len(downstreams) == 1 \
+                and self._canvas.get_component(downstreams[0])["obj"].component_name.lower() == "answer":
             return partial(self.stream_output, chat_mdl, prompt, retrieval_res)
 
         if "empty_response" in retrieval_res.columns: