Properly cleanup when the Widget is initialized by an implicit as_widget()

[cpsievert]

#191 introduced a problem where, if the widget that is initialized by an internal as_widget() call, then it doesn't get cleaned up.

For a minimal reprex of the issue, notice how clicking the button will keep inserting new plots

import plotly.express as px
from shiny.express import input, ui

from shinywidgets import render_plotly

ui.input_action_button("redraw", "Redraw")

@render_plotly
def plot():
    input.redraw()
    return px.histogram(
        px.data.tips(),
        x="total_bill",
    )