Make a presentation with two slides. Select slide 1 and drag a cube from basic objects palette into layer in the timeline tree without first expanding the layer -> cube is added as expected and the layer is expanded. Change to slide 2 and drag a cube from basic objects palette into layer in the timeline tree -> Crash.
If the layer is expanded on slide 1 when the first cube is dragged, then there is no crash on the second drag, either. The problem is likely in index conversion but why it only happens when the layer on slide 1 is not initially expanded is odd.