[chronojump] force sensor: fixed bad Chronojump exit if forceOtherThread not used

[Xavier de Blas <xaviblas src gnome org>]

commit 3359bf0ec8072d0b509ba76c0ea68bc43f518d3a
Author: Xavier de Blas <xaviblas gmail com>
Date:   Tue Oct 24 23:40:57 2017 +0200

    force sensor: fixed bad Chronojump exit if forceOtherThread not used

 src/gui/chronojump.cs |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/gui/chronojump.cs b/src/gui/chronojump.cs
index d270668..d791a32 100644
--- a/src/gui/chronojump.cs
+++ b/src/gui/chronojump.cs
@@ -2157,7 +2157,7 @@ public partial class ChronoJumpWindow
                        LogB.Information("cancelling force capture");
                        forceProcessCancel = true;
                }
-               if(forceOtherThread.IsAlive)
+               if(forceOtherThread != null && forceOtherThread.IsAlive)
                        forceOtherThread.Abort();
                if(portFSOpened)
                        portFS.Close();