Concurrent Modification when system is hardly loaded

[klesun]

It is in PlaybackTimer::iterate() on "for (Fraction key : keys) {}"... how something could change there at all ???

Opening a tightVNC viewer during playback led to CPU business, what caused bugs in our treads to show up:

Exception in thread "AWT-EventQueue-0" java.util.ConcurrentModificationException
    at java.util.TreeMap$NavigableSubMap$SubMapIterator.nextEntry(TreeMap.java:1699)
    at java.util.TreeMap$NavigableSubMap$SubMapKeyIterator.next(TreeMap.java:1776)
    at org.shmidusic.stuff.musica.PlaybackTimer.iterate(PlaybackTimer.java:64)
    at org.shmidusic.stuff.musica.PlaybackTimer.lambda$iterate$87(PlaybackTimer.java:76)