Hi, thanks everyone for your help !
After checking, it seems that I don’t have the latest version … Mine is dated on the 13/01/2018… However I downloaded it from the official website yesterday 
On the “About Seamly 2D” menu, when I click on “Check for updates”, the same bug appears and then the program crashes.
Could you help me to upgrade it please ?
I’m working with the following version of Ubuntu :
| Description: | Ubuntu 18.04.4 LTS |
|---|---|
| Release: | 18.04 |
| Codename: | bionic |
Here is a screenshot of my issue : as you can see, my only option is to click on the “X”.
Here is a screenshot of the “Preferences” menu :
Many thanks ! Inès