Bug - Crashing when opening multiple bookmarks in a new window.
Try upgrading to version 188.8.131.52 ( https://vivaldi.net/en-US/blogs/teamblog ) and see how that behaves.
I suspect the "open multiple bookmarks" function is just something that will have to be developed down the road a bit.
FYI … the latest snapshot did not resolve this problem. I can still recreate this issue.