Unable to add deb-src repository line from Other software tab using Add button

[Norbert]

Steps to reproduce:
1. Launch Software & Updates with `software-properties-gtk`
2. Switch to *Other Software* tab
3. Click *Add*
4. Enter correct deb-src repository line like `deb-src http://archive.ubuntu.com/ubuntu precise main universe multiverse`

Expected result:
* the entered repository line is added in the list of current tab (and to sources.list), user was asked about reloading repository information

Actual result:
* user asked to enter password, the entered repository line is ignored, user is not informed, dialog closed without any messages

Workaround:
* add binary repository `deb http://archive.ubuntu.com/ubuntu precise main universe multiverse` and then uncheck binaries and check source code mark

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: software-properties-gtk 0.96.20.8
ProcVersionSignature: Ubuntu 4.15.0-46.49~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-46-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.18
Architecture: amd64
CurrentDesktop: MATE
Date: Sat May 11 15:08:45 2019
InstallationDate: Installed on 2018-08-07 (276 days ago)
InstallationMedia: Ubuntu-MATE 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
PackageArchitecture: all
SourcePackage: software-properties
UpgradeStatus: No upgrade log present (probably fresh install)