Note that this is a pretty minimal fix. A potential future refactor to increase the robustness of the parser could be to use the null-object pattern and return a NullToken from
Commit e886115 introduced some tests that intend to iterate over strings that contain definitions of invalid source code and then assert that the parser does something sensible with these sources. However, there was a typo in e886115 causing the tests to not execute as expected: the invalid sources are iterated in the `source_ucli` variable. However, the parsing is done against the `source_ucl` variable which contains a known parsable source string!