These lines relate to 327-341 in _differential_evolution.py.  The default value for mutation when calling DifferentialEvolutionSolver is (0.5, 1).  This should make the solver use dithering for the mutation constant (DifferentialEvolutionSolver.scale). However, it's also possible to pass in a single value ((0, 2]) for the mutation constant, in which case dithering is switched off.  
On line 348 in _differential_evolution.py I assume that dithering is going to be switched off, then on the next line I switch it on if mutation is iterable and it's length is greater than 1.  In this test I check (137-138) that solver.dither is True when I pass valid input for dithering.
I don't think that solver.dither is going to be None though, so I'll remove that.