rest sink report -> panic: runtime error: invalid memory address or nil pointer dereference

[lesliechung88]

运行一段时间后，突然出现在这个情况：
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0xb2e74c]

goroutine 21 [running]:
github.com/emqx/kuiper/xstream/sinks.(*RestSink).send(0xc00012c460, 0xe9d3c0, 0xc00040e330, 0x10ef060, 0xc0001e2070, 0x1, 0x0)
/root/kuiper-edgex/xstream/sinks/rest_sink.go:283 +0x66c
github.com/emqx/kuiper/xstream/sinks.(*RestSink).Collect(0xc00012c460, 0x10eefe0, 0xc0001e60a0, 0xe56da0, 0xc000422340, 0x0, 0xc0000bf068)
/root/kuiper-edgex/xstream/sinks/rest_sink.go:180 +0x297
github.com/emqx/kuiper/xstream/nodes.doCollect(0x10df440, 0xc00012c460, 0xc000422320, 0x10eaf60, 0xc0000fa360, 0x3e8, 0xc000094360, 0x10eefe0, 0xc0001e60a0)
/root/kuiper-edgex/xstream/nodes/sink_node.go:181 +0x1af
github.com/emqx/kuiper/xstream/nodes.(*SinkNode).Open.func1.1(0xc000144d20, 0xc00016a2a0, 0x10eefe0, 0xc0001e60a0, 0x10ef060, 0xc0001e2070, 0xc0000b8ba8, 0xc0000b8bb0, 0xc0000b8b8e, 0xc0000b8ba0, ...)
/root/kuiper-edgex/xstream/nodes/sink_node.go:145 +0x3f8
created by github.com/emqx/kuiper/xstream/nodes.(*SinkNode).Open.func1
/root/kuiper-edgex/xstream/nodes/sink_node.go:108 +0x3c2

[jinfahua]

Thanks, it should be a bug... @ngjaying We should add error handling before determining the response code at https://github.com/emqx/kuiper/blob/master/xstream/sinks/rest_sink.go#L283