Can't start Synaptic from Additional Software that was itself started from tails-persistence-setup

As reported on tails-testers (“Opinion about Tails 4.0beta1”).

• Open Additional Software in 4.0~beta1.
• Click on “Synaptic Package Manager”.
• Nothing happens.

Related issues

• Related to #14544
• Related to #16273
• Blocks #16209

Original created by @sajolida on 16982 (Redmine)