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

@@ -103,10 +103,10 @@ def run():
     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
@@ -135,12 +135,13 @@ def run():
         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'])