Editing multiple ROIs can crash Slicer.

[allemangD]

Steps to reproduce:

1. Open PickAndPaint; select a model.
2. Create a new fiducial list (MarkupsFiducial)
3. Add a point
4. Create a new fiducial list (MarkupsFiducial_1)
5. Add a point
6. Select the first fiducial list (MarkupsFiducial)
7. Add a point. Slicer crashes.

Discovered while investigating #16.