I opened a plan and used the Add Special command to add Mercury to the list. Then immediately I used the delete key, hoping to remove Mercury from the list. AstroPlanner brought up a dialog saying that it had crashed and offering to submit a bug report. Unfortunately Windows claimed that the window was not responding, so I couldn't submit the report that way. Windows eventually decided that AstroPlanner wasn't running, but before it could delete the window, I copied down the information below. This problem is repeatable.

Exception Type: Nil Object Exception

1: Sub MainWindow.MainWindow.UpdateVisibility[MainWindow.MainWindow, PlanObject, boolean)
2: Sub MainWindow.MainWindow.UpdateVisibility[MainWindow.MainWindow, int32)
3: Sub MainWindow.MainWindow.CommitObjectListChange(MainWindow.MainWindow)

Fixed.