Fix for the poorly written User_functions.test_structure_add_atom GUI test.
This fixes one part of 2 of the bug #22772 (https://gna.org/bugs/?22772), the modelfree4 binary
issue and the User_functions GUI tests with wxPython 2.9 failures of the test suite.
The problem was that a list element was being set in the GUI test, but that element did not exist
yet. Somehow this worked in wxPython 2.8. But the bad code failed on wxPython 2.9.
