The best solution I have found so far is to periodically save your work, close the program, and start over from that point.
I do not believe that a new version of Ubuntu would help. As a matter of fact, I know of no one who has tested Seamly2D in 18.10 or 19.04 versions of Ubuntu yet. You are using the latest stable version of Seamly2D.
The “object by ID” probably represents a software bug and it will be fixed but likely only after it can be reproduced consistently.
If you can help with the quest to find a way to reproduce this error consistently that will be really helpful in getting it fixed. If you save your pattern file then save it to another name (like pattern-v1.val and pattern-v2.val and pattern-v3.val … etc) the chances are greater that you will have a version that requires only a few steps before the program fails. When that happens, it will be easy for someone to help you never encounter the problem again.