[chronojump] Fixed cancel and finish since last commit (forceThread)

[Xavier de Blas <xaviblas src gnome org>]

commit 4f0b0b6686ac86c5dbf38d8a54d6ef33f32fa68c
Author: Xavier de Blas <xaviblas gmail com>
Date:   Fri Mar 31 14:05:31 2017 +0200

    Fixed cancel and finish since last commit (forceThread)

 src/gui/chronojump.cs |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/gui/chronojump.cs b/src/gui/chronojump.cs
index 97e432c..ac37e99 100644
--- a/src/gui/chronojump.cs
+++ b/src/gui/chronojump.cs
@@ -3389,7 +3389,7 @@ public partial class ChronoJumpWindow
        {
                event_execute_ButtonCancel.Clicked -= new EventHandler(on_cancel_clicked);
 
-               if(forceThread.IsAlive)
+               if(forceThread != null && forceThread.IsAlive)
                {
                        LogB.Information("cancel clicked on force");
                        forceProcessCancel = true;
@@ -3425,7 +3425,7 @@ public partial class ChronoJumpWindow
                //to avoid doble finish or cancel while finishing
                hideButtons();
 
-               if(forceThread.IsAlive)
+               if(forceThread != null && forceThread.IsAlive)
                {
                        LogB.Information("finish clicked on force");
                        forceProcessFinish = true;