Assertion failed

[Fellow-baka]

There is some reproducible error (see screenshot) in Microsoft Windows [Version 10.0.14393] and Fityk 1.3.1.
How to reproduce:

1. Open some data.
2. Add any function.
3. Click on any parameter filed of this function.
4. While this field is active delete function with right click -> delete.
5. Click on list of available functions.
   Error appears only if there is only one function.