Replace SharpTreeNodeView InvalidOperationException with debug message.

SharpTreeView/SharpTreeNodeView.cs

@@ -7,6 +7,7 @@ using System.Windows;
 using System.Windows.Controls.Primitives;
 using System.Windows.Media;
 using System.ComponentModel;
+using System.Diagnostics;
 
 namespace ICSharpCode.TreeView
 {
@@ -145,8 +146,10 @@ namespace ICSharpCode.TreeView
 			else {
 				result -= 19;
 			}
-			if (result < 0)
+			if (result < 0) {
-				throw new InvalidOperationException();
+				Debug.WriteLine("SharpTreeNodeView.CalculateIndent() on node without correctly-set level");
+				return 0;
+			}
 			return result;
 		}
 	}