Fixing http://mutabit.com/repos.fossil/grafoscopio/tktview?name=ffb08a5973 as suggested by Udo.

src/Grafoscopio/GrafoscopioNode.class.st

@@ -126,14 +126,12 @@ GrafoscopioNode >> ancestors [
 	
 	currentNode := self.
 	ancestors := OrderedCollection new.
-	[currentNode level > 0] 
+	[ currentNode parent notNil and: [ currentNode level > 0 ] ]
 		whileTrue: [ 
 			ancestors add: currentNode parent.
-			currentNode := currentNode parent.].
+			currentNode := currentNode parent].
 	ancestors := ancestors reversed.
-   ^ ancestors 	
-	
-
+   ^ ancestors 
 ]
 
 { #category : #accessing }