chore: Avoids null pointer error when region_configs are null in tpf advanced_cluster (#3013)

* test: Add validation for null region_configs in advanced cluster schema

* fix: Add nil check for regionConfigs in ValidateList method

* test: Ensure TestAccAdvancedCluster_ValidationErrors is run

Author: EspenAlbert

internal/service/advancedclustertpf/schema_test.go

@@ -10,14 +10,18 @@ import (
 	"github.com/mongodb/terraform-provider-mongodbatlas/internal/testutil/acc"
 )
 
-func TestAdvancedCluster_ValidationErrors(t *testing.T) {
+func TestAccAdvancedCluster_ValidationErrors(t *testing.T) {
 	const (
 		projectID   = "111111111111111111111111"
 		clusterName = "test"
 	)
 	resource.ParallelTest(t, resource.TestCase{
 		ProtoV6ProviderFactories: acc.TestAccProviderV6Factories,
 		Steps: []resource.TestStep{
+			{
+				Config:      nullRegionConfigs, // can happen when using moved block, panic: runtime error: invalid memory address or nil pointer dereference
+				ExpectError: regexp.MustCompile("Missing Configuration for Required Attribute"),
+			},
 			{
 				Config:      acc.ConvertAdvancedClusterToSchemaV2(t, true, invalidRegionConfigsPriorities),
 				ExpectError: regexp.MustCompile("priority values in region_configs must be in descending order"),
@@ -154,3 +158,15 @@ resource "mongodbatlas_advanced_cluster" "test" {
 	}
 }
 `
+var nullRegionConfigs = `
+resource "mongodbatlas_advanced_cluster" "test" {
+	project_id     = "111111111111111111111111"
+	name           = "test-acc-tf-c-2670522663699021050"
+	cluster_type   = "REPLICASET"
+	backup_enabled = false
+
+	replication_specs = [{
+		region_configs = null
+	}]
+}
+`

internal/service/advancedclustertpf/validate_schema.go

@@ -95,7 +95,7 @@ func (v RegionSpecPriorityOrderDecreasingValidator) MarkdownDescription(_ contex
 func (v RegionSpecPriorityOrderDecreasingValidator) ValidateList(ctx context.Context, req validator.ListRequest, resp *validator.ListResponse) {
 	diags := &resp.Diagnostics
 	regionConfigs := newCloudRegionConfig20240805(ctx, req.ConfigValue, diags)
-	if diags.HasError() {
+	if diags.HasError() || regionConfigs == nil {
 		return
 	}
 	configs := *regionConfigs