Fix Key Error When Cleaning Up Event Handlers

[rmorshea]

Discussed in #639

^{Originally posted by jkrobicki February 2, 2022}
KeyError appers when I try to display components, however it works with version 0.34.0

[rmorshea]

The following will reproduce the error:

toggle_type = idom.Ref()
element_static_handler = StaticEventHandler()
component_static_handler = StaticEventHandler()

@idom.component
def Root():
    toggle, toggle_type.current = use_toggle(True)
    handler = element_static_handler.use(lambda: None)
    if toggle:
        return html.div(html.button({"onEvent": handler}))
    else:
        return html.div(SomeComponent())

@idom.component
def SomeComponent():
    handler = component_static_handler.use(lambda: None)
    return html.button({"onAnotherEvent": handler})

with idom.Layout(Root()) as layout:
    await layout.render()

    assert element_static_handler.target in layout._event_handlers
    assert component_static_handler.target not in layout._event_handlers

    toggle_type.current()
    await layout.render()

    assert element_static_handler.target not in layout._event_handlers
    assert component_static_handler.target in layout._event_handlers

    toggle_type.current()
    await layout.render()

    assert element_static_handler.target in layout._event_handlers
    assert component_static_handler.target not in layout._event_handlers