Obj null check to avoid failure on non-typed character like NULL as value for array type key/object

pkg/kapp/resources/mod_object_ref_set.go

@@ -44,6 +44,9 @@ func (t ObjectRefSetMod) apply(obj interface{}, path Path) error {
 		case part.ArrayIndex != nil:
 			switch {
 			case part.ArrayIndex.All != nil:
+				if obj == nil {
+					return nil
+				}
 				typedObj, ok := obj.([]interface{})
 				if !ok {
 					return fmt.Errorf("Unexpected non-array found: %T", obj)

test/e2e/templaterules_with_non_typed_array_fields_test.go

@@ -0,0 +1,64 @@
+// Copyright 2022 VMware, Inc.
+// SPDX-License-Identifier: Apache-2.0
+
+package e2e
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestTemplateRulesWithNonTypedArrayFields(t *testing.T) {
+	env := BuildEnv(t)
+	logger := Logger{}
+	kapp := Kapp{t, env.Namespace, env.KappBinaryPath, logger}
+	kubectl := Kubectl{t, env.Namespace, logger}
+
+	yaml := `
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: simple-app
+spec:
+  selector:
+    matchLabels:
+      simple-app: ""
+  template:
+    metadata:
+      labels:
+        simple-app: ""
+    spec:   
+      initContainers: null
+      containers:
+      - name: simple-app
+        image: docker.io/dkalinin/k8s-simple-app@sha256:4c8b96d4fffdfae29258d94a22ae4ad1fe36139d47288b8960d9958d1e63a9d0
+        env:
+        - name: HELLO_MSG
+          value: stranger
+        envFrom:
+---
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: simple-cm
+  annotations:
+    kapp.k14s.io/versioned: ""
+data:
+  hello_msg: carvel
+`
+
+	name := "test-templaterules-with-non-typed-array-fields"
+	cleanUp := func() {
+		kapp.Run([]string{"delete", "-a", name})
+	}
+
+	cleanUp()
+	defer cleanUp()
+	logger.Section("Initial deploy", func() {
+		kapp.RunWithOpts([]string{"deploy", "-a", name, "-f", "-"}, RunOpts{StdinReader: strings.NewReader(yaml)})
+
+		NewPresentClusterResource("deployment", "simple-app", env.Namespace, kubectl)
+		NewPresentClusterResource("configmap", "simple-cm-ver-1", env.Namespace, kubectl)
+	})
+}