Should not save and read system configuraton in /root/.synaptic/synaptic.conf

[golanguage]

After upgrading to Ubuntu dev release(the upgrade scripts prints some error about snap, but after upgrading the system is OK), Synaptic can not work , just prompt:

E: The value 'oracular' is invalid for APT::Default-Release as such a release is not available in the sources
E: _cache->open() failed, please report.

But all configurations under /etc/apt are correct.

At last, the problem is in /root/.synaptic/synaptic.conf, there is a line : DefaultDistro "oracular";

This a system configuration, should not read and write it from/into user config.