Strange slur is added when pressing S on the score's last note
Steps to reproduce:
1) Create a new score
2) Enter a note in the last measure so that there is no rest after it
3) Select the note, press S
Result: some strange slur is added:
If you deselect it you will probably never see it, but it exists, as the object debugger says.
OS: Debian GNU/Linux 9 (stretch), Arch.: x86_64, MuseScore version (64-bit): 3.0.0, revision: 64d3653