Errors in the `Calculator.bubble_widget()` method.

[jdebacker]

After creating two Calculator objects, named calc1 and calc2 and running:

bw = calc1.bubble_widget(calc2)
show(bw)

two warning messages result:

WARNING:bokeh.core.validation.check:W-1000 (MISSING_RENDERERS): Plot has no renderers: Figure(id='2156', ...)
WARNING:bokeh.core.validation.check:W-1000 (MISSING_RENDERERS): Plot has no renderers: Figure(id='2212', ...)

And no Bokeh plot object is displayed. This error the results in no plot object being created seems to have arisen with more recent versions of Bokeh, although the widget has never come out looking exactly as one would like it.

An example of instantiating and running these Calculator objects is provided in PSL_demo.ipynb (in the documentation here.