fix: null pointer error with running subtasks prematurely

mentioned in commit 69af8c40

mentioned in merge request docker/nginx-ingress!202

mentioned in merge request docker/softether-vpnsrv!195 (merged)

mentioned in merge request docker/seafile-cli!194 (merged)

mentioned in merge request docker/vizier!52 (merged)

mentioned in merge request docker/softether-vpnsrv!197

mentioned in merge request docker/nginx-ingress!204