Error when assigning color

A recent error report we got:

```
Details:
Traceback (most recent call last):

  File "vanillaBase.py", line 514, in action_
    self.callback(sender)

  File "plugin.py", line 421, in CheckBox_Callback
    layer.userData["com.hugojourdan.ColorFlow"][color]= True

TypeError: '__NSFrozenDictionaryM' object does not support item assignment
```