Only disable other sites if they are enabled.

debian/ngcp-panel.postinst

@@ -22,10 +22,14 @@ a2enmod ssl perl
 a2enmod dump_io || echo "Missing some optional Apache module(s), ignoring."
 
 # disable debian's default Apache site
-a2dissite default
+if [ -e /etc/apache2/sites-enabled/default ]; then
+    a2dissite default
+fi
 
 # disable old ngcp-www-admin
-a2dissite ngcp-www-admin
+if [ -e /etc/apache2/sites-enabled/ngcp-www-admin ]; then
+    a2dissite ngcp-www-admin
+fi
 
 # reload perl and/or apache modules
 if [ -x /etc/init.d/apache2 ] ; then