added null check

src/Compilers/Core/Portable/Generated/Operations.xml.Generated.cs

-// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
 using System;
 using System.Collections.Generic;
@@ -13,7 +13,7 @@ namespace Microsoft.CodeAnalysis.Semantics
     internal abstract partial class BaseAddressOfExpression : Operation, IAddressOfExpression
     {
         protected BaseAddressOfExpression(SemanticModel semanticModel, SyntaxNode syntax, ITypeSymbol type, Optional<object> constantValue) :
-                    base(OperationKind.AddressOfExpression, semanticModel, syntax, type, constantValue)
+            base(OperationKind.AddressOfExpression, semanticModel, syntax, type, constantValue)
         {
         }
 

src/Compilers/Core/Portable/Operations/Operation.cs

@@ -97,7 +97,8 @@ public static IOperation CreateOperationNone(SemanticModel semanticModel, Syntax
 
         public static T SetParentOperation<T>(T operation, IOperation parent) where T : IOperation
         {
-            (operation as Operation).SetParentOperation(parent);
+            // operation can be null
+            (operation as Operation)?.SetParentOperation(parent);
             return operation;
         }
 
@@ -154,7 +155,8 @@ public override void Accept(OperationVisitor visitor)
         {
             void EnqueueChildOperations(Queue<IOperation> queue, IOperation parent)
             {
-                foreach (var o in parent.Children)
+                // children can return null
+                foreach (var o in parent.Children.WhereNotNull())
                 {
                     queue.Enqueue(o);
                 }