[gbrainy] Fixes null exception

[Jordi Mas <jmas src gnome org>]

commit d8b0bc2ef350d02d7d02892b4cddea0710143824
Author: Jordi Mas <jmas softcatala org>
Date:   Sun Feb 27 17:32:58 2011 +0100

    Fixes null exception

 src/Core/Main/Xml/CodeEvaluation.cs |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/Core/Main/Xml/CodeEvaluation.cs b/src/Core/Main/Xml/CodeEvaluation.cs
index df4b79b..ca433c1 100644
--- a/src/Core/Main/Xml/CodeEvaluation.cs
+++ b/src/Core/Main/Xml/CodeEvaluation.cs
@@ -117,7 +117,8 @@ namespace gbrainy.Core.Main.Xml
 			Regex regex;
 			Match match;
 
-			if (String.IsNullOrEmpty (str))
+			if (String.IsNullOrEmpty (str) ||
+				String.IsNullOrEmpty (stored_vars))
 				return str;
 
 			regex = new Regex (exp, RegexOptions.IgnoreCase);