[chronojump] Fixed crash on right click treeview persons with no persons

[Xavier de Blas <xaviblas src gnome org>]

commit 7c4ecb0ddea4be3ed93735f56fc1cb3164e638a8
Author: Xavier de Blas <xaviblas gmail com>
Date:   Mon Jan 25 18:01:03 2021 +0100

    Fixed crash on right click treeview persons with no persons

 src/gui/app1/chronojump.cs | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/src/gui/app1/chronojump.cs b/src/gui/app1/chronojump.cs
index 48b030f9..4780f520 100644
--- a/src/gui/app1/chronojump.cs
+++ b/src/gui/app1/chronojump.cs
@@ -1071,11 +1071,12 @@ public partial class ChronoJumpWindow
                //Log.WriteLine("IS " + tvEvent.ExpandState);
        }
 
-       private void on_treeview_button_release_event (object o, ButtonReleaseEventArgs args) {
+       private void on_treeview_button_release_event (object o, ButtonReleaseEventArgs args)
+       {
                Gdk.EventButton e = args.Event;
                Gtk.TreeView myTv = (Gtk.TreeView) o;
                if (e.Button == 3) {
-                       if(myTv == treeview_persons) {
+                       if(myTv == treeview_persons && currentPerson != null) {
                                treeviewPersonsContextMenu(currentPerson);
                        } else if(myTv == treeview_jumps) {
                                if (myTreeViewJumps.EventSelectedID > 0) {