Fix nullptr after listener consumer closed #968

Job	Run time
Formatting Check	1m 40s
Run unit tests	22m 53s
Lint	29m 45s
Build the Wireshark dissector (ubuntu-latest)	2m 11s
Build CPP Client on Windows x64	33m 47s
Build CPP Client on Windows x86	31m 30s
Build Alpine x86_64	16m 32s
Build Deb x86_64	10m 14s
Build CPP Client on macOS with static dependencies	9m 4s
Build RPM x86_64	12m 3s
Build with the C++20 standard	5m 40s
Check Completion	3s
	2h 55m 22s

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