What steps will reproduce the problem?
1.Open the log viewer
2.Close the log viewer
3.Open the log viewer
4.Change Font settings

What is the expected output? What do you see instead?
After changing the settings, i get the attached exception

I suggest to remove the PropertyChangeListener on disposal of a LogViewer 
instance:
@Override
public void dispose() {
   viewer.removeListeners();  // new method
   storeAllCurrentlyOpenFiles();
   super.dispose();
}

The new method in LogFileViewer looks like: 
public void removeListeners() {
   if (propertyChangeListener != null) {
      store.removePropertyChangeListener(propertyChangeListener);
   }
}