Can't add a route without gateway in Network Manager VPN.IPv4 window

Dear Ubuntu programers,

I would like report about possibly bug in Network Manager and edit VPN connection.
In edit window for VPN and IPv4 I can't add a route without gateway, because the Gateway field is red highlight. And I can't click to the button "Apply", because it is grey (see the attached picture).

I don't know if this a bug or not.
But I don't know gateway for VPN connection, because the VPN connection uses PPP.

If I add the route via nmcli, it works well:

==================================================
# nmcli c e Skylab
nmcli> set ipv4.routes 192.168.240.0/24
nmcli> print
...
ipv4.routes: { ip = 192.168.240.0/24 }
...
nmcli> save

# ip r
192.168.240.0/24 dev ppp0 proto static scope link metric 50
==================================================

I can't set this setting via GUI Network Manager.

I know, that is workaround via nmcli, but the way is not good for normal users. When Ubuntu is for "normal" users mainly, the GUI is better way.

$ lsb_release -rd
Description: Ubuntu 18.04 LTS
Release: 18.04

$ apt-cache policy gnome-control-center
gnome-control-center:
  Installed: 1:3.28.1-0ubuntu1.18.04.1
  Candidate: 1:3.28.1-0ubuntu1.18.04.1
  Version table:
 *** 1:3.28.1-0ubuntu1.18.04.1 500
        500 http://cz.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1:3.28.1-0ubuntu1 500
        500 http://cz.archive.ubuntu.com/ubuntu bionic/main amd64 Packages

Thank you.
Yours Faithfully
Radomir.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: gnome-control-center 1:3.28.1-0ubuntu1.18.04.1
ProcVersionSignature: Ubuntu 4.15.0-23.25-generic 4.15.18
Uname: Linux 4.15.0-23-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Fri Jun 22 09:22:30 2018
ExecutablePath: /usr/bin/gnome-control-center
InstallationDate: Installed on 2018-06-08 (13 days ago)
InstallationMedia: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
SourcePackage: gnome-control-center
UpgradeStatus: No upgrade log present (probably fresh install)