FIX:will not display 500 error dialog when sending print

jira:[for network error]

Change-Id: Id5fbd2182d3c1948f67382d81f8bf558daffa1c6
(cherry picked from commit 5b7cc2495cdfe8889cbddc9390bf096f72e52604)

src/slic3r/GUI/GUI_App.cpp

@@ -1581,6 +1581,10 @@ void GUI_App::init_networking_callbacks()
                     m_server_error_dialog = new NetworkErrorDialog(mainframe);
                 }
 
+                if(plater()->get_select_machine_dialog() && plater()->get_select_machine_dialog()->IsShown()){
+                    return;
+                }
+
                 if (m_server_error_dialog->m_show_again) {
                     return;
                 }