Ensure a session context is present when widgets are closed

[cpsievert]

Closes #170

The reason this didn't come up in testing is because I was using an app that used invalidate_later() for invalidation, and it that case, the session is actually present .on_invalidate(). Whereas, if a reactive effect gets invalidated in the normal way, it apparently doesn't exist at .on_invalidate() time.

This seems like a bug in Shiny, but since this is such a bad regression in shinywidgets, I'm gonna do this patch fix for now:

from shiny import reactive
from shiny.express import input, ui
from shiny.reactive import get_current_context, invalidate_later
from shiny.session import get_current_session


@reactive.effect
def _():
    invalidate_later(1)

    # Prints the session, as expected
    ctx = get_current_context()
    ctx.on_invalidate(lambda: print(get_current_session()))


ui.input_action_button("btn", "Print session")


@reactive.effect
def _():
    input.btn()

    # Prints None?!?
    ctx = get_current_context()
    ctx.on_invalidate(lambda: print("Button click: ", get_current_session()))

[schloerke]

Shinylive app link