Changing stream keys stops working when Delta F is added

Once the Delta F is plotted, selecting which keys are plotted is not possible. Choosing the keys button produces the following error: 
   ```
   [...]magpy/gui/dialogclasses.py", line 3807, in OnGetKeys
    exec('dlg.'+elem+'CheckBox.SetValue(True)')
    File "<string>", line 1, in <module>
   AttributeError: 'StreamSelectKeysDialog' object has no attribute 'dfCheckBox' 
```