There's a problem in dual-pane mode where activites outlive
DelegateBase instances that are tied to fragments. AlertDialogs, being
bound to the MainActivity, can sometimes outlive the delegates that
create them, meaning the 'this' referred to from closures bound to
onClick() handlers can come to be invalid (e.g. referencing a removed
fragment). So add a global registry of current DelegateBase instances
by class, and from onClick() handlers fetch and use the current
instance instead of the 'this' that's bound.
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