[chronojump] Fix to on_combo_select_contacts_top_changed at end of import on windows (at least on innolab virtual

[Xavier de Blas <xaviblas src gnome org>]

commit bf452977f0c09b5d6de3de53ce2bc6e93b8de603
Author: Xavier de Blas <xaviblas gmail com>
Date:   Tue Nov 30 18:44:26 2021 +0100

    Fix to on_combo_select_contacts_top_changed at end of import on windows (at least on innolab virtual 
machine)

 src/gui/app1/chronojump.cs | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/src/gui/app1/chronojump.cs b/src/gui/app1/chronojump.cs
index 5f624f52b..d37bb8b00 100644
--- a/src/gui/app1/chronojump.cs
+++ b/src/gui/app1/chronojump.cs
@@ -2413,6 +2413,15 @@ public partial class ChronoJumpWindow
        private void on_combo_select_contacts_top_changed (object o, EventArgs args)
        {
                LogB.Information("on_combo_select_contacts_top_changed");
+
+               ComboBox combo = o as ComboBox;
+               if(combo == null || UtilGtk.ComboGetActive(combo) == "")
+               {
+                       LogB.Information(" ...but is null or empty.");
+                       //happens at end of import, at least on windows at innolab virtual machine
+                       return;
+               }
+
                if(current_mode == Constants.Modes.JUMPSSIMPLE)
                        on_combo_select_jumps_changed(o, args);
                else if(current_mode == Constants.Modes.JUMPSREACTIVE)