Fix #333: Unable to save WiX file.

Allow saving of an updated WiX file when it has been edited in the
Setup - Files view.

src/AddIns/BackendBindings/WixBinding/Project/Src/Gui/OpenTextEditors.cs

@@ -37,8 +37,10 @@ namespace ICSharpCode.WixBinding
 		{
 			foreach (IViewContent view in workbench.ViewContentCollection) {
 				ITextEditor textEditor = view.GetService<ITextEditor>();
-				if (AreFileNamesEqual(view.PrimaryFileName, document.FileName)) {
-					return textEditor;
+				if (textEditor != null) {
+					if (AreFileNamesEqual(view.PrimaryFileName, document.FileName)) {
+						return textEditor;
+					}
 				}
 			}
 			return null;

src/AddIns/BackendBindings/WixBinding/Test/PackageFiles/OpenTextEditorsTestFixture.cs

@@ -18,6 +18,7 @@
 
 using System;
 using ICSharpCode.SharpDevelop;
+using ICSharpCode.SharpDevelop.Editor;
 using ICSharpCode.WixBinding;
 using NUnit.Framework;
 using WixBinding.Tests.Utils;
@@ -37,7 +38,7 @@ namespace WixBinding.Tests.PackageFiles
 		{
 			SD.InitializeForUnitTests();
 			existingTextEditor = new MockTextEditor();
-			MockTextEditorViewContent viewContent = new MockTextEditorViewContent();
+			var viewContent = new MockTextEditorViewContent();
 			viewContent.TextEditor = existingTextEditor;
 			viewContent.SetFileName(@"d:\projects\test\file.wxs");
 			
@@ -64,5 +65,19 @@ namespace WixBinding.Tests.PackageFiles
 			unknownDocument.FileName = @"d:\unknown-file.wxs";
 			Assert.IsNull(openEditors.FindTextEditorForDocument(unknownDocument));
 		}
+		
+		[Test]
+		public void FindTextEditorForDocument_FirstViewContentHasNoTextEditorAndNoPrimaryFileName_DoesNotThrowNullReferenceException()
+		{
+			var viewContent = new MockViewContent();
+			viewContent.PrimaryFile = null;
+			workbench.ViewContentCollection.Add(viewContent);
+			var unknownDocument = new WixDocument();
+			unknownDocument.FileName = @"d:\unknown-file.wxs";
+			
+			ITextEditor textEditor = openEditors.FindTextEditorForDocument(unknownDocument);
+			
+			Assert.IsNull(textEditor);
+		}
 	}
 }

src/AddIns/BackendBindings/WixBinding/Test/Utils/MockViewContent.cs

@@ -65,6 +65,7 @@ namespace WixBinding.Tests.Utils
 		
 		public OpenedFile PrimaryFile {
 			get { return primaryFile; }
+			set { primaryFile = value; }
 		}
 		
 		public FileName PrimaryFileName {