deactivating repository prevents synaptic use

[H V Schmidt]

Bug Description

Not a new bug, New to me, but I see others have seen same.
If you uncheck a repository which is also indicated as the preferred distro branch under preferences, the application immediately throws up an error and closes upon updating repository database. Reopening just repeats the error+close routine. I assume this would likely not occur if the preferred branch were changed before repo change.

Error text:
E;The value 'stretch-backports' is invalid for APT::Default-Release as such a release is not available in the sources
E:_cache->open() failed, please report.
(Note second bug, also in the triaged state) - Copy and paste of error dialog boxes in synaptic is disabled/non-functioning

System: X86_64_Linux_gnu
OS: Devuan ascii (Deb "stretch" fork)
application synaptic-0.84.2 stable

Steps to reproduce:
1. Add a new repository to list and check to include and allow local database update.
2. Change preferred branch under preferences to pull from new repo.
3. Download the latest greatest app of your choice from new repo (Kicad 5.0.2 btw)
4. Uncheck new repo since not needed any longer.
5. Click to initiate local database update and watch while synaptic poops on your pillow.

Suggestions to fix:
1. Allow postponement of database update after repo change to give user time to make additional changes.
2. Reword dialog box around preferred branch type selection. "Preferred release", and even "default-release" don't indicate ONLY release option.
3. A little error checking between repo selection cross referenced with release type to either self correct or pop up a warning to give user a chance to fix before proceeding.
4. Allow release branch preference to pull from more stable or secure branches if indicated branch is no longer available.

BTW, Reporting here since unbuntu is indicated as the maintainer of synaptic front end app.