Fixed a null reference issue during the process of Get-AzContext -ListAvailable (#24865)

* Fixed a null reference issue during the process of Get-AzContext -ListAvailable [#24854]

* Update src/Accounts/Accounts/Context/GetAzureRMContext.cs

Author: BethanyZhou

src/Accounts/Accounts/ChangeLog.md

@@ -19,6 +19,7 @@
 -->
 
 ## Upcoming Release
+* Fixed a null reference issue during the process of `Get-AzContext -ListAvailable` [#24854].
 * Supported interactive subscription selection for user login flow. See more details at [Announcing a new login experience with Azure PowerShell and Azure CLI
 ](https://techcommunity.microsoft.com/t5/azure-tools-blog/announcing-a-new-login-experience-with-azure-powershell-and/ba-p/4109357)
 * Added config `LoginExperienceV2` to allow customer to switch the default behavior of context selection back. Check the help document of `Update-AzConfig` for more details.

src/Accounts/Accounts/Context/GetAzureRMContext.cs

@@ -104,8 +104,8 @@ public override void ExecuteCmdlet()
                 var profile = DefaultProfile as AzureRmProfile;
                 if (profile != null && profile.Contexts != null)
                 {
-                    WritePSAzureContext(profile.Contexts.Select(context => ToPSAzureContext(context.Value, context.Key)).
-                        OrderBy(context => context.Tenant.Id).ThenBy(context => context.Subscription.Name));
+                    WritePSAzureContext(profile.Contexts.Select(context => ToPSAzureContext(context.Value, context.Key))?.Where(context => null != context) // remove null contexts
+                        ?.OrderBy(context => context?.Tenant?.Id ?? "").ThenBy(context => context?.Subscription?.Name ?? ""));
                 }
 
             }
@@ -129,10 +129,17 @@ public override void ExecuteCmdlet()
             }
         }
 
+        /// <summary>
+        /// Convert AzureContext to PSAzureContext with given name
+        /// Notice that returned context may be null
+        /// </summary>
+        /// <param name="azureContext"></param>
+        /// <param name="name"></param>
+        /// <returns></returns>
         private PSAzureContext ToPSAzureContext(IAzureContext azureContext, string name)
         {
             var context = new PSAzureContext(azureContext);
-            if (name != null)
+            if (null != name && null != context)
             {
                 context.Name = name;
             }