How to reproduce:

• Load MRHead sample
• Go to the Editor, activate the LevelTracingEffect
• Click on an edge voxel
• Slicer crashes

The root cause of the problem is that the itkLevelTracingImageFilter accesses neighbours of the provided seed point without checking if they exist (inside the image).

Proposed fix: In the Modules\Scripted\EditorLib\LevelTracingEffect.py file in the preview(self,xy) function the index checking should be change to something like this:
if ijk[index] < 1 or ijk[index] >= dimensions[index]-1:
return