Fix null reference

Author: Lei Jin

src/Accounts/Accounts/Context/ClearAzureRmContext.cs

@@ -74,7 +74,7 @@ void ClearContext(AzureRmProfile profile, RMProfileClient client)
                 var contexts = profile.Contexts.Values;
                 foreach (var context in contexts)
                 {
-                    tokenCacheProvider?.ClearCache(context.Environment.ActiveDirectoryAuthority);
+                    tokenCacheProvider?.ClearCache(context?.Environment?.ActiveDirectoryAuthority);
                     client.TryRemoveContext(context);
                 }
 

src/Accounts/Accounts/Context/GetAzureRMContext.cs

@@ -84,7 +84,10 @@ public override void ExecuteCmdlet()
                     var defaultProfile = DefaultProfile as AzureRmProfile;
                     if (defaultProfile != null && string.Equals(AzureSession.Instance?.ARMContextSaveMode, "CurrentUser"))
                     {
-                        AzureSession.Instance.SetProperty(AzureSession.Property.Environment, DefaultContext.Environment.Name);
+                        if (!string.IsNullOrEmpty(DefaultContext?.Environment?.Name))
+                        {
+                            AzureSession.Instance.SetProperty(AzureSession.Property.Environment, DefaultContext.Environment.Name);
+                        }
                         defaultProfile.RefreshContextsFromCache(_cmdletContext);
                     }
                 }

src/Accounts/Accounts/Context/RemoveAzureRmContext.cs

@@ -90,7 +90,7 @@ public override void ExecuteCmdlet()
                                         }
                                         else
                                         {
-                                            if (!tokenCacheProvider.TryRemoveAccount(removedContext.Account.Id, removedContext.Environment.ActiveDirectoryAuthority))
+                                            if (!tokenCacheProvider.TryRemoveAccount(removedContext.Account.Id, removedContext?.Environment?.ActiveDirectoryAuthority))
                                             {
                                                 WriteWarning(string.Format(Resources.NoContextsRemain, removedContext.Account.Id));
                                             }

src/Accounts/Authentication/Factories/AuthenticationFactory.cs

@@ -523,10 +523,10 @@ public void RemoveUser(IAzureAccount account, IAzureEnvironment environment)
                             // make best effort to remove credentials
                         }
 
-                        RemoveFromTokenCache(account, environment.ActiveDirectoryAuthority);
+                        RemoveFromTokenCache(account, environment?.ActiveDirectoryAuthority);
                         break;
                     case AzureAccount.AccountType.User:
-                        RemoveFromTokenCache(account, environment.ActiveDirectoryAuthority);
+                        RemoveFromTokenCache(account, environment?.ActiveDirectoryAuthority);
                         break;
                 }
             }