Hi Kathia,
I see the problem looking in the code - basically when you draw a Slider without specifying its size, it looks up the Window's units in a dictionary called defaultSizes to find what size to set it to by default. Currently, this dictionary only has values for 'norm':
defaultSizes = {'norm': [1.0, 0.1]}
I’ll add some more presets for the next release, but for the time being you should be able to get around this bug by specifying a size for your Slider so that it doesn’t have to check for defaults.
Thanks,
Todd