Fix null on dispose error
Fix null on dispose error

CenturyLinkCloud/kafka-net,BDeus/KafkaNetClient,bridgewell/kafka-net,martijnhoekstra/kafka-net,PKRoma/kafka-net,geffzhang/kafka-net,aNutForAJarOfTuna/kafka-net,gigya/KafkaNetClient,nightkid1027/kafka-net,Jroland/kafka-net,EranOfer/KafkaNetClient