The source of the information contained by this panel when the information transitions from another panel to this one. By default it is
null, indicating the information does not transition from any other particular panel.
The information transition starts when a panel is closed and the information is visually transferred to this panel, like when dismissing by swipe or by pressing an accept button. For example, an incoming call notification may transition to a main process panel showing the call progress. In this example, NotificationPanel would be provided as source.
This field may be used by the system UI to properly visualize the transition. For example, a main process panel that normally enters immediately may instead use a delay to neatly align with another panel's exit animation.
This property must be set before the system UI shows it for it to have effect. Typically this means it should be set before the panel is passed to Frontend.addPanel.
When this property is set, it may also be relevant to set the transitionDestination of the source panel as well.