[chronojump] Fixed crash on new code selecting bars on encoder by clicking

[Xavier de Blas <xaviblas src gnome org>]

commit 437f02f04553f8feffeb7d762ca4915741a1eeee
Author: Xavier de Blas <xaviblas gmail com>
Date:   Wed Jan 29 18:51:06 2020 +0100

    Fixed crash on new code selecting bars on encoder by clicking

 src/gui/encoderGraphObjects.cs | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/src/gui/encoderGraphObjects.cs b/src/gui/encoderGraphObjects.cs
index 9a9a4a06..98d45a30 100644
--- a/src/gui/encoderGraphObjects.cs
+++ b/src/gui/encoderGraphObjects.cs
@@ -788,6 +788,9 @@ public class EncoderGraphDoPlot
 
        public int FindBarInPixel (double pixel)
        {
+               if(encoderBarsLimits == null)
+                       return -1;
+
                return encoderBarsLimits.FindBarInPixel(pixel);
        }