The help file for the Tremolo.ms object says the following:
To create a tremolo, first create two RestChords of the desired duration. For whole, half and quarter note tremolos, the rest duration should be half of the note's duration. For eighth tremolos, the note duration should be quarter and the rest duration sixteenth.
I chose this approach for eighth note tremolos since there is no easy way to hide the flag on eighth notes 
Here is a fixed version of that measure. Also, please note that you should mute the notes on each side of the tremolo object if you are using playback.
!NoteWorthyComposerClip(2.751,Single)
|Chord|Dur:8th|Pos:-9,-2|Opts:Stem=Up,Beam=First
|Note|Dur:8th|Pos:-5|Opts:Stem=Up,Beam
|Chord|Dur:8th|Pos:-9,-2|Opts:Stem=Up,Beam
|Note|Dur:8th|Pos:-5|Opts:Stem=Up,Beam=End
|Spacer|Width:200
|RestChord|Dur:16th|Opts:Stem=Down,ArticulationsOnStem,Muted,HideRest|Dur2:4th|Pos2:-7|Color:1
|Spacer|Width:300|Color:1
|User|Tremolo.ms|Pos:-8|Color:1
|RestChord|Dur:16th|Opts:Stem=Down,ArticulationsOnStem,Muted,HideRest|Dur2:4th|Pos2:-2|Color:1
|Spacer|Width:375|Color:1
|Chord|Dur:4th|Pos:-9,-2
!NoteWorthyComposerClip-End