antrea-controller panic when running NetworkPolicy e2e

[tnqn]

Describe the bug

The e2e test below detected a panic in antrea-controller:
https://github.com/antrea-io/antrea/actions/runs/8180733633/job/22369493710?pr=6073

E0307 00:39:19.704309      13 runtime.go:79] Observed a panic: "invalid memory address or nil pointer dereference" (runtime error: invalid memory address or nil pointer dereference)
goroutine 601 [running]:
k8s.io/apimachinery/pkg/util/runtime.logPanic({0x238c1a0?, 0x40ce950})
	k8s.io/apimachinery@v0.26.4/pkg/util/runtime/runtime.go:75 +0x85
k8s.io/apimachinery/pkg/util/runtime.HandleCrash({0x0, 0x0, 0xc001e7a6b0?})
	k8s.io/apimachinery@v0.26.4/pkg/util/runtime/runtime.go:49 +0x6b
panic({0x238c1a0?, 0x40ce950?})
	runtime/panic.go:914 +0x21f
antrea.io/antrea/pkg/controller/networkpolicy.(*NetworkPolicyController).getAppliedToWorkloads(0xc0006ea000, 0xc001b829b0)
	antrea.io/antrea/pkg/controller/networkpolicy/networkpolicy_controller.go:1384 +0xb0
antrea.io/antrea/pkg/controller/networkpolicy.(*NetworkPolicyController).syncAppliedToGroup(0xc0006ea000, {0xc001e2f200, 0x13})
	antrea.io/antrea/pkg/controller/networkpolicy/networkpolicy_controller.go:1305 +0x34f
antrea.io/antrea/pkg/controller/networkpolicy.(*NetworkPolicyController).processNextAppliedToGroupWorkItem(0xc0006ea000)
	antrea.io/antrea/pkg/controller/networkpolicy/networkpolicy_controller.go:1059 +0x158
antrea.io/antrea/pkg/controller/networkpolicy.(*NetworkPolicyController).appliedToGroupWorker(0xc0006ea000)
	antrea.io/antrea/pkg/controller/networkpolicy/networkpolicy_controller.go:962 +0x9b
k8s.io/apimachinery/pkg/util/wait.BackoffUntil.func1(0x30?)
	k8s.io/apimachinery@v0.26.4/pkg/util/wait/wait.go:157 +0x33
k8s.io/apimachinery/pkg/util/wait.BackoffUntil(0x0?, {0x2ba9060, 0xc0014716b0}, 0x1, 0xc0007b96e0)
	k8s.io/apimachinery@v0.26.4/pkg/util/wait/wait.go:158 +0xaf
k8s.io/apimachinery/pkg/util/wait.JitterUntil(0x0?, 0x3b9aca00, 0x0, 0x0?, 0x0?)
	k8s.io/apimachinery@v0.26.4/pkg/util/wait/wait.go:135 +0x7f
k8s.io/apimachinery/pkg/util/wait.Until(0x0?, 0x0?, 0x0?)
	k8s.io/apimachinery@v0.26.4/pkg/util/wait/wait.go:92 +0x1e
created by antrea.io/antrea/pkg/controller/networkpolicy.(*NetworkPolicyController).Run in goroutine 255
	antrea.io/antrea/pkg/controller/networkpolicy/networkpolicy_controller.go:953 +0x578
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x50 pc=0x1eded30]

Logs:
e2e-kind-encap-non-default.tar.gz.zip