CAPO Panics when adding ManagedSecurityGroups

[grosenba]

/kind bug

What steps did you take and what happened:
[A clear and concise description of what the bug is.]
https://kubernetes.slack.com/archives/CFKJB65G9/p1714481726268869?thread_ts=1714372906.792849&cid=CFKJB65G9

2024/04/30 12:53:40 http: panic serving 100.90.194.192:35079: runtime error: invalid memory address or nil pointer dereference

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

2024/04/30 12:53:40 http: panic serving 100.90.194.192:35079: runtime error: invalid memory address or nil pointer dereference
goroutine 5384333 [running]:
net/http.(*conn).serve.func1()
	/usr/local/go/src/net/http/server.go:1898 +0xbe
panic({0x1db33e0?, 0x3646f90?})
	/usr/local/go/src/runtime/panic.go:770 +0x132
sigs.k8s.io/cluster-api-provider-openstack/pkg/webhooks.(*openStackClusterWebhook).ValidateUpdate(0xc0005b0ac0?, {0xc001a62000?, 0xfee?}, {0x2415f10?, 0xc00142f908?}, {0x2415f10, 0xc00142ec88})
	/workspace/pkg/webhooks/openstackcluster_webhook.go:124 +0x3bf
sigs.k8s.io/controller-runtime/pkg/webhook/admission.(*validatorForType).Handle(_, {_, _}, {{{0xc000e3ff80, 0x24}, {{0xc0018c8de0, 0x1f}, {0xc001776560, 0x7}, {0xc001776570, ...}}, ...}})
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.16.3/pkg/webhook/admission/validator_custom.go:102 +0x70b
sigs.k8s.io/controller-runtime/pkg/webhook/admission.(*Webhook).Handle(_, {_, _}, {{{0xc000e3ff80, 0x24}, {{0xc0018c8de0, 0x1f}, {0xc001776560, 0x7}, {0xc001776570, ...}}, ...}})
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.16.3/pkg/webhook/admission/webhook.go:169 +0x1ee
sigs.k8s.io/controller-runtime/pkg/webhook/admission.(*Webhook).ServeHTTP(0xc00067eff0, {0x7fd5de5c3d78, 0xc00172cf50}, 0xc000df0900)
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.16.3/pkg/webhook/admission/http.go:98 +0xc32
sigs.k8s.io/controller-runtime/pkg/webhook/internal/metrics.InstrumentedHook.InstrumentHandlerInFlight.func1({0x7fd5de5c3d78, 0xc00172cf50}, 0xc000df0900)
	/go/pkg/mod/github.com/prometheus/client_golang@v1.17.0/prometheus/promhttp/instrument_server.go:60 +0xcb
net/http.HandlerFunc.ServeHTTP(0x241c730?, {0x7fd5de5c3d78?, 0xc00172cf50?}, 0x41a025?)
	/usr/local/go/src/net/http/server.go:2166 +0x29
github.com/prometheus/client_golang/prometheus/promhttp.InstrumentHandlerCounter.func1({0x241c730?, 0xc0000d0b60?}, 0xc000df0900)
	/go/pkg/mod/github.com/prometheus/client_golang@v1.17.0/prometheus/promhttp/instrument_server.go:147 +0xc3
net/http.HandlerFunc.ServeHTTP(0x6ffd86?, {0x241c730?, 0xc0000d0b60?}, 0x412025?)
	/usr/local/go/src/net/http/server.go:2166 +0x29
github.com/prometheus/client_golang/prometheus/promhttp.InstrumentHandlerDuration.func2({0x241c730, 0xc0000d0b60}, 0xc000df0900)
	/go/pkg/mod/github.com/prometheus/client_golang@v1.17.0/prometheus/promhttp/instrument_server.go:109 +0xc2
net/http.HandlerFunc.ServeHTTP(0xc0000c3860?, {0x241c730?, 0xc0000d0b60?}, 0x10?)
	/usr/local/go/src/net/http/server.go:2166 +0x29
net/http.(*ServeMux).ServeHTTP(0x412025?, {0x241c730, 0xc0000d0b60}, 0xc000df0900)
	/usr/local/go/src/net/http/server.go:2683 +0x1ad
net/http.serverHandler.ServeHTTP({0x2414138?}, {0x241c730?, 0xc0000d0b60?}, 0x6?)
	/usr/local/go/src/net/http/server.go:3137 +0x8e
net/http.(*conn).serve(0xc000a2f5f0, {0x242d778, 0xc000300fc0})
	/usr/local/go/src/net/http/server.go:2039 +0x5e8
created by net/http.(*Server).Serve in goroutine 165
	/usr/local/go/src/net/http/server.go:3285 +0x4b4

Environment:

• Cluster API Provider OpenStack version (Or git rev-parse HEAD if manually built): 0.10.0
• Cluster-API version: 1.7.0
• OpenStack version:
• Minikube/KIND version:
• Kubernetes version (use kubectl version):
• OS (e.g. from /etc/os-release):

[EmilienM]

/cc EmilienM