Unclear error when use geom_rect with discrete scales

```python
from lets_plot import *
LetsPlot.setup_html()

ggplot() + geom_rect(xmin=0, xmax=10, ymin=0, ymax=5) + scale_discrete(['x','y'])
```

Output:
>Internal error: NoSuchElementException : Can't create DoubleSpan: the input is empty or contains NULLs.

