Widgets initialized in a `reactive.effect` are unexpectedly removed

[cpsievert]

Related to #174

Clicking the button in the app below will (expectedly) add a new marker, but (unexpectedly) remove the previously added marker.

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

from shinywidgets import render_widget

ui.input_action_button("add", "Add Marker")

m = Map(center=(5, 5), zoom=5)

@render_widget
def map():
    return m

@reactive.effect
@reactive.event(input.add)
def _():
    marker = Marker(location=(input.add(), input.add()))
    m.add(marker)