Do not crash when writing changes if old configuration is nil

src/lib/y2network/sysconfig/config_writer.rb

@@ -36,9 +36,7 @@ class ConfigWriter
       # @param config     [Y2Network::Config] Configuration to write
       # @param old_config [Y2Network::Config] Old configuration
       def write(config, old_config = nil)
-        return unless config.routing
-
-        write_ip_forwarding(config.routing)
+        write_ip_forwarding(config.routing) if config.routing
         write_interface_changes(config, old_config)
 
         # update /etc/sysconfig/network/routes file

src/lib/y2network/sysconfig/dns_writer.rb

@@ -45,21 +45,23 @@ def write(dns, old_dns)
       # @return [String] Sendmail update script (included in "sendmail" package)
       SENDMAIL_UPDATE_PATH = "/usr/lib/sendmail.d/update".freeze
 
-      def update_sysconfig_dhcp(config, old_config)
-        if old_config.dhcp_hostname == config.dhcp_hostname
+      # @param dns [Y2Network::DNS] DNS configuration
+      # @param old_dns [Y2Network::DNS] Old DNS configuration
+      def update_sysconfig_dhcp(dns, old_dns)
+        if old_dns.nil? || old_dns.dhcp_hostname == dns.dhcp_hostname
           log.info("No update for /etc/sysconfig/network/dhcp")
           return
         end
 
         Yast::SCR.Write(
           Yast::Path.new(".sysconfig.network.dhcp.DHCLIENT_SET_HOSTNAME"),
-          config.dhcp_hostname == :any ? "yes" : "no"
+          dns.dhcp_hostname == :any ? "yes" : "no"
         )
         Yast::SCR.Write(Yast::Path.new(".sysconfig.network.dhcp"), nil)
 
         # Clean-up values from ifcfg-* values
         Y2Network::Sysconfig::InterfaceFile.all.each do |file|
-          value = file.interface == config.dhcp_hostname ? "yes" : nil
+          value = file.interface == dns.dhcp_hostname ? "yes" : nil
           file.load
           next if file.dhclient_set_hostname == value
           file.dhclient_set_hostname = value