[Mod]clear thread handler when downloading data failed

vnpy/app/cta_backtester/engine.py

@@ -329,14 +329,14 @@ class BacktesterEngine(BaseEngine):
             symbol, exchange, Interval(interval), start, end
         )
 
-        if not data:
+        if data:
+            database_manager.save_bar_data(data)
+            self.write_log(f"{vt_symbol}-{interval}历史数据下载完成")
+        else:
             self.write_log(f"数据下载失败，无法获取{vt_symbol}的历史数据")
 
-        database_manager.save_bar_data(data)
-
         # Clear thread object handler.
         self.thread = None
-        self.write_log(f"{vt_symbol}-{interval}历史数据下载完成")
 
     def start_downloading(
         self,