I had just added a large number of images to my plan, and had been using the arrow keys on the keyboard to scroll through them. That all seemed to work fine. But then when I was done, I clicked the close box in the upper right corner of the window. AsttroPlanner started flashing many windows that were probably intended to tell me something had gone wrong. Eventually AP exited without giving me a chance to actually read any of the windows or submit a bug report. AP's log file contained about 3 dozen messages like the one below. Note that I had not been using ASCOM.

Hilary
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Exception Type: Nil Object Exception

1: Function MainWindow.MainWindow.BGPaint( MainWindow.MainWindow, Graphics, int32, int32 ) as boolean
2: Function MainWindow.MainWindow.ObjectList_CellBackgroundPaint( MainWindow.MainWindow, Graphics, int32, int32 ) as boolean
3: Function ObjectListbox.Event_CellBackgroundPaint( Graphics, int32, int32 ) as boolean
4: Function StripedListBox.Event_CellBackgroundPaint( Graphics, int32, int32 ) as boolean
5: Sub OLEObject.Constructor( string )
6: Sub ASCOM.Create()
7: Sub ASCOM.Constructor( TelescopeMount, string )
8: Function ASCOMMount.ASCOMobj.Get( int32 ) as ASCOM
9: Sub ASCOMMount.Destroy()
10: Sub MountCollection.DestroyMount( string )
11: Sub MountCollection.ReleaseMount( MainWindow.MainWindow, string )
12: Sub MainWindow.MainWindow.Event_Close( MainWindow.MainWindow )

Fixed.