[chronojump] Fixed a potential crash on buttonpress a cairobars graph when graph is not created

[Xavier de Blas <xaviblas src gnome org>]

commit ce5537af1f5b90521c4821a44be5879f9d0ed7da
Author: Xavier de Blas <xaviblas gmail com>
Date:   Thu Mar 24 12:45:53 2022 +0100

    Fixed a potential crash on buttonpress a cairobars graph when graph is not created

 src/gui/eventExecute.cs | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/src/gui/eventExecute.cs b/src/gui/eventExecute.cs
index 47ebe0a20..9bdc089f6 100644
--- a/src/gui/eventExecute.cs
+++ b/src/gui/eventExecute.cs
@@ -2486,6 +2486,9 @@ public abstract class CairoPaintBarsPre
 
        public int FindBarInPixel (double pixel)
        {
+               if(cb == null)
+                       return -1;
+
                return cb.FindBarInPixel(pixel);
        }
 }