I had this exact issue after disabling a source in Other Software. I disabled the third source you see here, “Automatically generated…”:
And then was unable to launch the Driver Manager / Software Sources app. To fix it, I re-enabled the source manually by editing /etc/apt/sources.list.d/ubuntu.sources. I found the entry for the disabled source:
Types: deb
URIs: http://us.archive.ubuntu.com/ubuntu
Suites: plucky plucky-updates plucky-backports
Components: main universe restricted multiverse
Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg
Enabled: no
And I changed “Enabled: no” to “Enabled: yes”. After that, I was able to launch the application as normal.