Use the local context variable that was retrieved earlier instead of
action->priv->context which is always NULL at this point since we
reset it right before. This ensures we invoke the callback on the
correct context.
And make sure that the wait message still has a reference when setting done
otherwise it is unref before calling the context invoke function
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9013>
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