Selecting a Level throws a NullPointerException

```
Exception in thread "JavaFX Application Thread" java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
....
Caused by: java.lang.NullPointerException
	at cz.gresak.keyboardeditor.EditorController.setSpecialComboBoxToCurrentKey(EditorController.java:269)
	at cz.gresak.keyboardeditor.EditorController.comboLevelChanged(EditorController.java:253)
```