fix: Fixes nil pointer dereference if advanced_configuration update…

… fails in `mongodbatlas_cluster` (#2139)

.changelog/2139.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+resource/mongodbatlas_cluster: Fixes nil pointer dereference if `advanced_configuration` update fails
+```

internal/service/cluster/resource_cluster.go

@@ -12,17 +12,19 @@ import (
 	"strings"
 	"time"
 
+	matlas "go.mongodb.org/atlas/mongodbatlas"
+
 	"github.com/hashicorp/terraform-plugin-sdk/v2/diag"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/retry"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema"
 	"github.com/hashicorp/terraform-plugin-sdk/v2/helper/validation"
+	"github.com/spf13/cast"
+
 	"github.com/mongodb/terraform-provider-mongodbatlas/internal/common/constant"
 	"github.com/mongodb/terraform-provider-mongodbatlas/internal/common/conversion"
 	"github.com/mongodb/terraform-provider-mongodbatlas/internal/common/validate"
 	"github.com/mongodb/terraform-provider-mongodbatlas/internal/config"
 	"github.com/mongodb/terraform-provider-mongodbatlas/internal/service/advancedcluster"
-	"github.com/spf13/cast"
-	matlas "go.mongodb.org/atlas/mongodbatlas"
 )
 
 const (
@@ -926,9 +928,9 @@ func resourceMongoDBAtlasClusterUpdate(ctx context.Context, d *schema.ResourceDa
 		if aclist, ok1 := ac.([]any); ok1 && len(aclist) > 0 {
 			advancedConfReq := expandProcessArgs(d, aclist[0].(map[string]any))
 			if !reflect.DeepEqual(advancedConfReq, matlas.ProcessArgs{}) {
-				argResp, _, err := conn.Clusters.UpdateProcessArgs(ctx, projectID, clusterName, advancedConfReq)
+				_, _, err := conn.Clusters.UpdateProcessArgs(ctx, projectID, clusterName, advancedConfReq)
 				if err != nil {
-					return diag.FromErr(fmt.Errorf(errorAdvancedConfUpdate, clusterName+argResp.DefaultReadConcern, err))
+					return diag.FromErr(fmt.Errorf(errorAdvancedConfUpdate, clusterName, err))
 				}
 			}
 		}