Fix NullReferenceException for incorrect SubstitutionGroup in XML schema.

Handle XML schema that defines an element that uses a SubstitutionGroup
element but does not define the corresponding element.

src/AddIns/DisplayBindings/XmlEditor/Project/Src/XmlSchemaCompletion.cs

@@ -347,7 +347,9 @@ namespace ICSharpCode.XmlEditor
 		{
 			if (!element.SubstitutionGroup.IsEmpty) {
 				XmlSchemaElement substitutedElement = FindElement(element.SubstitutionGroup);
-				return GetElementAsComplexType(substitutedElement);
+				if (substitutedElement != null) {
+					return GetElementAsComplexType(substitutedElement);
+				}
 			}
 			return null;
 		}

src/AddIns/DisplayBindings/XmlEditor/Test/Schema/SubstitutionGroupTests.cs

@@ -0,0 +1,38 @@
+// Copyright (c) AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
+
+using System;
+using ICSharpCode.XmlEditor;
+using NUnit.Framework;
+
+namespace XmlEditor.Tests.Schema
+{
+	[TestFixture]
+	public class SubstitutionGroupTests : SchemaTestFixtureBase
+	{
+		string namespaceURI = "http://www.w3.org/1999/XSL/Transform";
+		
+		protected override string GetSchema()
+		{
+			return
+				"<xs:schema " +
+				"    xmlns:xs=\"http://www.w3.org/2001/XMLSchema\"" +
+				"    xmlns:xsl=\"http://www.w3.org/1999/XSL/Transform\"" +
+				"    targetNamespace=\"http://www.w3.org/1999/XSL/Transform\"" +
+				"    elementFormDefault=\"qualified\">\r\n" +
+				"    <xs:element name=\"stylesheet\" substitutionGroup=\"xsl:transform\"/>\r\n" +
+				"</xs:schema>";
+		}
+		
+		[Test]
+		public void GetChildElementCompletion_ParentElementIsSubstitutionGroupButNoCorrespondingElementInSchema_NullReferenceExceptionIsNotThrown()
+		{
+			var path = new XmlElementPath();
+			path.AddElement(new QualifiedName("stylesheet", namespaceURI));
+			
+			XmlCompletionItemCollection items = SchemaCompletion.GetChildElementCompletion(path);
+			
+			Assert.AreEqual(0, items.Count);
+		}
+	}
+}

src/AddIns/DisplayBindings/XmlEditor/Test/XmlEditor.Tests.csproj

@@ -168,6 +168,7 @@
     <Compile Include="Schema\SingleElementSchemaTestFixture.cs" />
     <Compile Include="Schema\ElementWithAttributeSchemaTestFixture.cs" />
     <Compile Include="Schema\NestedElementSchemaTestFixture.cs" />
+    <Compile Include="Schema\SubstitutionGroupTests.cs" />
     <Compile Include="Schema\XsltSchemaTests.cs" />
     <Compile Include="Schema\TwoElementSchemaTestFixture.cs" />
     <Compile Include="Schema\ReferencedElementsTestFixture.cs" />