Perform null check before accessing it (#16552)

* Perform null check before accessing it 

Perform the null check on RouteTable.Routes before accessing it with SingleOrDefault.

* Update ChangeLog.md

* Update ChangeLog.md

fix change log

* Update ChangeLog.md

Co-authored-by: Jin Lei <[email protected]>
Co-authored-by: Yunchi Wang <[email protected]>

Author: 3 people

src/Network/Network/ChangeLog.md

@@ -19,6 +19,7 @@
 --->
 
 ## Upcoming Release
+* Fixed `ArgumentNullException` in `Add-AzureRmRouteConfig` when `RouteTable.Routes` is null.
 
 ## Version 4.16.0
 * Added support for retrieving the state of packet capture even when the provisioning state of the packet capture was failure
@@ -69,7 +70,6 @@
     - Also updated cmdlet to add the property for configuring ExclusionManagedRuleSet within Exclusions
         - `New-AzApplicationGatewayFirewallPolicyExclusion`
 * Bug Fix in Application Gateway Trusted Client Certificate cmdlets to load the entire cert chain from file.
-
 ## Version 4.12.0
 * Support for Sku, ScaleUnits parameters of BastionHost resource.
     - `New-AzBastion`

src/Network/Network/Generated/RouteTable/Route/AddAzureRmRouteConfigCommand.cs

@@ -78,18 +78,19 @@ public partial class AddAzureRmRouteConfigCommand : NetworkBaseCmdlet
 
         public override void Execute()
         {
-
+             // Routes
+            if (this.RouteTable.Routes == null)
+            {
+                this.RouteTable.Routes = new List<PSRoute>();
+            }
+            
             var existingRoute = this.RouteTable.Routes.SingleOrDefault(resource => string.Equals(resource.Name, this.Name, System.StringComparison.CurrentCultureIgnoreCase));
             if (existingRoute != null)
             {
                 throw new ArgumentException("Route with the specified name already exists");
             }
 
-            // Routes
-            if (this.RouteTable.Routes == null)
-            {
-                this.RouteTable.Routes = new List<PSRoute>();
-            }
+           
 
             var vRoutes = new PSRoute();