Allow for the model_id property to still be accessed after the widget is closed

[cpsievert]

This problem was first discovered via #174

Here is a small reprex of the problem.

from ipyleaflet import Map, Marker
from shiny import reactive
import shiny.express

from shinywidgets import render_widget

map = Map(zoom=2)

@render_widget
def map_out():
    reactive.invalidate_later(1)
    marker = Marker(location=(0, 0))
    map.add(marker)
    return map

After the 1st invalidation, you'll get:

AttributeError: 'NoneType' object has no attribute 'comm_id'

This happens because .add() reads of the .model_id property of every Widget instance (i.e., Marker()) that has ever been added to it. And, in recent versions of shinywidgets, we now .close() widgets when the reactive context they were initialized in get invalidated. When a widget gets closed, normally reading .model_id will result in error since that property reads .comm.comm_id, but .comm is None.

The workaround here is come up with a "mock" comm object that we add after close that allows for the .model_id property to still be read.