geom_crossbar: an error occurs if the mapped values of aesthetics are not populated.

[RYangazov]

Example:

from lets_plot import *
LetsPlot.setup_html()

data = {
    'drink': ['coffee','tea','water', 'milk'],
    'mean': [3, 4, 6, 2],
    'upper': [1, 5, 7, 4],
    'lower': [6, 3, 2, 1]
}

ggplot(data, aes(x='drink', middle='mean', ymin='lower', ymax='upper', color='mean')) + geom_crossbar()

Result:
AttributeError: 'NoneType' object has no attribute 'props'