[chronojump] Minor fix since last commits

[Xavier de Blas <xaviblas src gnome org>]

commit 3f56cef9a07930f46cedc8bf86730ee6676a7530
Author: Xavier de Blas <xaviblas gmail com>
Date:   Fri Jul 1 14:56:23 2016 +0200

    Minor fix since last commits

 src/gui/eventExecute.cs |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)
---
diff --git a/src/gui/eventExecute.cs b/src/gui/eventExecute.cs
index c98460f..9a8036b 100644
--- a/src/gui/eventExecute.cs
+++ b/src/gui/eventExecute.cs
@@ -916,6 +916,10 @@ public partial class ChronoJumpWindow
                int ancho=drawingarea.Allocation.Width;
                int alto=drawingarea.Allocation.Height;
                
+               //fix problem on show graph at Chronojump start
+               if(event_execute_drawingarea == null || event_execute_pixmap == null)
+                       return;
+
                UtilGtk.ErasePaint(event_execute_drawingarea, event_execute_pixmap);
                //writeMarginsText(eventGraph.sessionMAXAtSQL, minValue, alto);