Comment 8 for bug 889565

just googled the error and found this

It appears that it's related to accessibility settings. I was able to fix this problem on my system by opening Universal Access, enabling then disabling the screen reader, then opening Synaptic again.

Try:

gsettings set org.gnome.desktop.interface toolkit-accessibility false

It worked for me.

https://bugs.launchpad.net/synaptic/+bug/607605/comments/6

Is there any way to do this with the new update without having to put it in manually?