[chronojump] Fixed crash on ErrorWindow HideAndNull

[Xavier de Blas <xaviblas src gnome org>]

commit 0b83bd7a98d4c7e54ed4c530e945b5937ae8e0a6
Author: Xavier de Blas <xaviblas gmail com>
Date:   Tue Sep 1 11:47:41 2015 +0200

    Fixed crash on ErrorWindow HideAndNull

 src/gui/error.cs |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/src/gui/error.cs b/src/gui/error.cs
index 09ba2ca..23d28cd 100644
--- a/src/gui/error.cs
+++ b/src/gui/error.cs
@@ -224,8 +224,10 @@ public class ErrorWindow
        }
        
        public void HideAndNull() {
-               ErrorWindowBox.error_window.Hide();
-               ErrorWindowBox = null;
+               if(ErrorWindowBox != null) {
+                       ErrorWindowBox.error_window.Hide();
+                       ErrorWindowBox = null;
+               }
        }
 
        ~ErrorWindow() {}