request help: traffic-split 插件提示错误

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Issue description

apisix版本 2.15
controller 版本 1.8.0
k8s 版本 1.26
apisix 集群是独立部署（部署了3个节点）， 非k8s内部署
应用的部署控制使用了argo-rollout 的金丝雀部署功能
一些路由会出现以下错误， 每次出现时只要把 controller 重启就能恢复正常

错误日志:
2024/09/06 09:21:10 [error] 373320#373320: *19527373058 lua entry thread aborted: runtime error: /data/www/gateway_apisix/apisix/plugins/traffic-split.lua:205: attempt to index field 'upstream' (a string value)
stack traceback:
coroutine 0:
/data/www/gateway_apisix/apisix/plugins/traffic-split.lua: in function 'create_obj_fun'
/data/www/gateway_apisix/apisix/core/lrucache.lua:95: in function 'lrucache'
/data/www/gateway_apisix/apisix/plugins/traffic-split.lua:267: in function 'phase_func'
/data/www/gateway_apisix/apisix/plugin.lua:934: in function 'run_plugin'
/data/www/gateway_apisix/apisix/init.lua:461: in function 'http_access_phase'
access_by_lua(nginx.conf:329):2: in main chunk, client: 223.72.83.215, server: _, request: "POST /dma/de HTTP/2.0", host: "app.xxxxx.com", referrer: "https://app.xxxxx.com/"

Environment

• your apisix-ingress-controller version (output of apisix-ingress-controller version --long):
• your Kubernetes cluster version (output of kubectl version):
• if you run apisix-ingress-controller in Bare-metal environment, also show your OS version (uname -a):