Storing the pointer to the fade effect only works as long as we control
all the code that can modify the effects of an actor, but this is not
the case in the presence of extensions. Apparently some extension
manages to remove the effect, which leads to fade_effect pointing to
some invalid address by the time dispose is called, leading to a crash.
This instead uses clutter_actor_get_effect() to get the effect when it
is needed to avoid crashing in these situations.
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6521
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2974>
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