[chronojump] Fixed a crash from today commits

[Xavier de Blas <xaviblas src gnome org>]

commit 19bfad5653bae4927f8284df5ca954d70915f963
Author: Xavier de Blas <xaviblas gmail com>
Date:   Thu Sep 26 17:04:43 2019 +0200

    Fixed a crash from today commits

 src/gui/forceSensor.cs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/gui/forceSensor.cs b/src/gui/forceSensor.cs
index 781b5fe1..e4d196f1 100644
--- a/src/gui/forceSensor.cs
+++ b/src/gui/forceSensor.cs
@@ -444,7 +444,7 @@ public partial class ChronoJumpWindow
                button_force_sensor_analyze_options.Sensitive = false;
                button_force_sensor_analyze_analyze.Sensitive = false;
                button_delete_last_test.Sensitive = false;
-               if(force_capture_drawingarea != null)
+               if(force_capture_drawingarea != null && force_capture_pixmap != null)
                        UtilGtk.ErasePaint(force_capture_drawingarea, force_capture_pixmap);
 
                label_force_sensor_value_max.Text = "";