[PATCH installer] gui: only run next button callback if there is one

[Maximiliano Sandoval <m.sandoval@proxmox.com>]

When testing the GUI installer via `make check-pve` the traceroute
command can time out, during this time the callback is not yet defined
and pressing the Next button will result in warnings in the CLI.

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
---
 proxinstall | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/proxinstall b/proxinstall
index 49b1612..91fcc4a 100755
--- a/proxinstall
+++ b/proxinstall
@@ -165,7 +165,7 @@ sub create_main_window {
     $next_btn->signal_connect(
         clicked => sub {
             Proxmox::Install::reset_last_display_change();
-            $gtk_state->{next_btn_callback}->();
+            $gtk_state->{next_btn_callback}->() if $gtk_state->{next_btn_callback};
         },
     );
     $cmdbox->pack_end($next_btn, 0, 0, 10);
-- 
2.47.3