Fix possible crash in ICSharpCode.Reporting preview WpfView

src/AddIns/Misc/Reporting/ICSharpCode.Reporting/Src/PageBuilder/ExportColumns/ExportText.cs

@@ -55,7 +55,9 @@ namespace ICSharpCode.Reporting.PageBuilder.ExportColumns
 		public Font Font {get;set;}
 		
 		
-		public string Text {get;set;}
+		private string _text;
+
+		public string Text { get { return _text ?? string.Empty; } set { _text = value; } }
 		
 		public string FormatString {get;set;}