Fixes `InvalidThreadForMutationException`s that pop up when
disconnecting/reconnecting tablets during the game's operation. In those
cases the value change callback executes from an OpenTabletDriver
thread.
`SliderSelectionBlueprint.OnDeselected()` would expire the
`ControlPointVisualiser` on deselection, leading to its removal from the
blueprint and eventual disposal, but still kept a separate reference to
said visualiser in another field.
This could lead to that stale reference to a disposed child getting read
in `ReceivePositionalInputAt()`, crashing quite a ways down over at the
framework side on futilely trying to compute the bounding box of a
drawable with no parent.