fix NullReferenceException if IEnumerable contains no elements

src/AddIns/Debugger/Debugger.AddIn/TreeModel/ArrayRangeNode.cs

@@ -14,7 +14,7 @@ namespace Debugger.AddIn.TreeModel
 		public static IEnumerable<TreeNode> LazyGetChildNodesOfArray(TreeNode parent, Expression expression, ArrayDimensions dimensions)
 		{
 			if (dimensions.TotalElementCount == 0)
-				return new TreeNode[] { new TreeNode(null, "(empty)", null, null, parent, null) };
+				return new TreeNode[] { new TreeNode(null, "(empty)", null, null, parent, _ => null) };
 			
 			return new ArrayRangeNode(parent, expression, dimensions, dimensions).ChildNodes;
 		}

src/AddIns/Debugger/Debugger.AddIn/TreeModel/ChildNodesOfObject.cs

@@ -117,9 +117,9 @@ namespace Debugger.AddIn.TreeModel
 				error = e;
 			}
 			if (error != null) {
-				yield return new TreeNode(null, "(error)", error.Message, null, null, null);
+				yield return new TreeNode(null, "(error)", error.Message, null, null, _ => null);
 			} else if (count == 0) {
-				yield return new TreeNode(null, "(empty)", null, null, null, null);
+				yield return new TreeNode(null, "(empty)", null, null, null, _ => null);
 			} else {
 				for(int i = 0; i < count; i++) {
 					string imageName;

src/AddIns/Debugger/Debugger.AddIn/TreeModel/TreeNode.cs

@@ -118,6 +118,8 @@ namespace Debugger.AddIn.TreeModel
 		public TreeNode(IImage iconImage, string name, string text, string type, TreeNode parent, Func<TreeNode, IEnumerable<TreeNode>> childNodes)
 			: this(parent)
 		{
+			if (childNodes == null)
+				throw new ArgumentNullException("childNodes");
 			this.iconImage = iconImage;
 			this.name = name;
 			this.text = text;