fix(acns): npe issue for acns transit encryption flag

Signed-off-by: Quang Nguyen <nguyenquang@microsoft.com>

Author: nddq

src/aks-preview/HISTORY.rst

@@ -12,6 +12,10 @@ To release a new version, please select a new version number (usually plus 1 to
 Pending
 +++++++
 
+18.0.0b14
++++++++
+* Fixed NPE issue for `--acns-transit-encryption-type`.
+
 18.0.0b13
 +++++++
 * Add option `--enable-http-proxy` to `az aks update`.

src/aks-preview/azext_aks_preview/managed_cluster_decorator.py

@@ -3028,7 +3028,9 @@ def set_up_network_profile(self, mc: ManagedCluster) -> ManagedCluster:
                         )
                     )
                 else:
-                    acns.security.transit_encryption.type = acns_transit_encryption_type
+                    acns.security.transit_encryption = self.models.AdvancedNetworkingSecurityTransitEncryption(
+                        type=acns_transit_encryption_type
+                    )
             network_profile.advanced_networking = acns
         return mc
 
@@ -4127,7 +4129,9 @@ def update_acns_in_network_profile(self, mc: ManagedCluster) -> ManagedCluster:
                         )
                     )
                 else:
-                    acns.security.transit_encryption.type = acns_transit_encryption_type
+                    acns.security.transit_encryption = self.models.AdvancedNetworkingSecurityTransitEncryption(
+                        type=acns_transit_encryption_type
+                    )
             mc.network_profile.advanced_networking = acns
         return mc