[ui] Graph.canExpandForLoop: fix corner case

Author: fabiencastan

meshroom/ui/graph.py

@@ -748,6 +748,8 @@ def duplicateNodesFrom(self, nodes):
     def canExpandForLoop(self, currentEdge):
         """ Check if the list attribute can be expanded by looking at all the edges connected to it. """
         listAttribute = currentEdge.src.root
+        if not listAttribute:
+            return False
         srcIndex = listAttribute.index(currentEdge.src)
         allSrc = [e.src for e in self._graph.edges.values()]
         for i in range(len(listAttribute)):