[MusicXML export] Invalid MusicXML will be generated if exported sheet contains tied cue notes
To reproduce this issue, one may export the uploaded sheet to MusicXML, generated MusicXML is invalid as it does not satisfy XSD of MusicXML.
Users could workaround this by delete the disallowed < tie> element in affected < note>s. It is pretty annoying though.
MusicXML does not allow a cue note has < tie> child element. The reason is that < tie> is designed for sound information, but cue notes are silent. Simple have a < tied> element under < notations> will be enough.
The fix should consists of a few lines of codes only. Correct logic will be, when parsing a tied note, does not add < tie> element into < note> when this note is grace and issue will be resolved.