Kuiper server quits when a rule is drop

[jinfahua]

It quit with following error stack trace.

bin/server
Serving Kuiper server on port 20498
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x146bb2c]

goroutine 60 [running]:
engine/xstream/extensions.(*MQTTSource).Close(0xc000103880, 0x164d240, 0xc0001326c0, 0xc005528f38, 0x1)
	/Users/rockyjin/Downloads/workspace/edge/src/kuiper/xstream/extensions/mqtt_source.go:150 +0x8c
engine/xstream/nodes.(*SourceNode).Open.func1(0xc00002bc40, 0x164d240, 0xc0001326c0, 0xc0055380e0, 0xc00a8bc000)
	/Users/rockyjin/Downloads/workspace/edge/src/kuiper/xstream/nodes/source_node.go:45 +0x1a2
created by engine/xstream/nodes.(*SourceNode).Open
	/Users/rockyjin/Downloads/workspace/edge/src/kuiper/xstream/nodes/source_node.go:28 +0x11c