Gateway做群组定时推送，20m左右后报这个错。一般是什么原因造成的

Stack  trace:
#0  \vendor\workerman\workerman\Connection\TcpConnection.php(336):  Workerman\Protocols\Websocket::encode(NULL,  Object(Workerman\Connection\TcpConnection))
#1  \vendor\workerman\workerman\Protocols\Websocket.php(167):  Workerman\Connection\TcpConnection->send(NULL)
#2  \vendor\workerman\workerman\Connection\TcpConnection.php(608):  Workerman\Protocols\Websocket::input('\x89\x80}c\xE8\xB2',  Object(Workerman\Connection\TcpConnection))
#3  :  Workerman\Connection\TcpConnection->baseRead(Resource  id  #1683)
#4  \vendor\workerman\workerman\Events\Select.php(289):  call_user_func_array(Array,  Array)
#5  \vendor\workerman\workerman\Worker.php(22  in  \vendor\workerman\workerman\Protocols\Websocket.php  on  line  236"
2018-07-18  16:45:49  pid:1  Worker  process  terminated  with  ERROR:  E_ERROR  "Uncaught  Exception:  You  can't  send(NULL)  to  client,  you  need  to  convert  it  to  a  string.    in  \vendor\workerman\workerman\Protocols\Websocket.php:236

是做心跳的没有接受到数据