升级2.1后，使用mongodb查询数据后，每隔一段时间报错：

Error: Call to a member function prepare() on null in /Users/roc/WebManWork/webman/vendor/illuminate/database/Connection.php:407
Stack trace:

0 /Users/roc/WebManWork/webman/vendor/illuminate/database/Connection.php(812): Illuminate\Database\Connection->Illuminate\Database{closure}('select 1', Array)

1 /Users/roc/WebManWork/webman/vendor/illuminate/database/Connection.php(779): Illuminate\Database\Connection->runQueryCallback('select 1', Array, Object(Closure))

2 /Users/roc/WebManWork/webman/vendor/illuminate/database/Connection.php(398): Illuminate\Database\Connection->run('select 1', Array, Object(Closure))

3 /Users/roc/WebManWork/webman/vendor/webman/database/src/DatabaseManager.php(46): Illuminate\Database\Connection->select('select 1')

4 /Users/roc/WebManWork/webman/vendor/workerman/coroutine/src/Pool.php(291): Webman\Database\DatabaseManager->Webman\Database{closure}(Object(MongoDB\Laravel\Connection))

5 /Users/roc/WebManWork/webman/vendor/workerman/coroutine/src/Pool.php(123): Workerman\Coroutine\Pool->checkConnections()

6 /Users/roc/WebManWork/webman/vendor/workerman/workerman/src/Events/Event.php(285): Workerman\Coroutine\Pool->Workerman\Coroutine{closure}()

7 /Users/roc/WebManWork/webman/vendor/workerman/workerman/src/Events/Event.php(144): Workerman\Events\Event->safeCall(Object(Closure), Array)

8 [internal function]: Workerman\Events\Event->Workerman\Events{closure}(-1, 1, NULL)

9 /Users/roc/WebManWork/webman/vendor/workerman/workerman/src/Events/Event.php(250): EventBase->loop()

10 /Users/roc/WebManWork/webman/vendor/workerman/workerman/src/Worker.php(1742): Workerman\Events\Event->run()

11 /Users/roc/WebManWork/webman/vendor/workerman/workerman/src/Worker.php(1544): Workerman\Worker::forkOneWorkerForLinux(Object(Workerman\Worker))

12 /Users/roc/WebManWork/webman/vendor/workerman/workerman/src/Worker.php(1524): Workerman\Worker::forkWorkersForLinux()

13 /Users/roc/WebManWork/webman/vendor/workerman/workerman/src/Worker.php(593): Workerman\Worker::forkWorkers()

14 /Users/roc/WebManWork/webman/vendor/workerman/webman-framework/src/support/App.php(143): Workerman\Worker::runAll()

15 /Users/roc/WebManWork/webman/start.php(5): support\App::run()

16 {main}