[simple-scan/cherry-pick-1294fa21] Disable possibility of scanning if no device is found, to avoid unnecessary errors

[Bartosz <bkosiorek src gnome org>]

commit cfd6e8121784e9367f04cafe19a6bf06489c5b66
Author: Bartosz Kosiorek <gang65 poczta onet pl>
Date:   Fri Apr 10 23:02:59 2020 +0000

    Disable possibility of scanning if no device is found, to avoid unnecessary errors
    
    
    (cherry picked from commit 1294fa21cbdd7f63ee4958b15c4120f870b060f9)

 src/app-window.vala | 2 ++
 1 file changed, 2 insertions(+)
---
diff --git a/src/app-window.vala b/src/app-window.vala
index 446353d..30adac9 100644
--- a/src/app-window.vala
+++ b/src/app-window.vala
@@ -245,6 +245,7 @@ public class AppWindow : Gtk.ApplicationWindow
 
     private void update_scan_status ()
     {
+        scan_button.set_sensitive(false);
         if (!have_devices)
         {
             status_primary_label.set_text (/* Label shown when searching for scanners */
@@ -254,6 +255,7 @@ public class AppWindow : Gtk.ApplicationWindow
         }
         else if (get_selected_device () != null)
         {
+            scan_button.set_sensitive(true);
             status_primary_label.set_text (/* Label shown when detected a scanner */
                                            _("Ready to Scan"));
             status_secondary_label.set_text (get_selected_device_label ());