fixed some reference errors

nltk/draw/tree.py

@@ -175,6 +175,7 @@ class TreeSegmentWidget(CanvasWidget):
         self.update(self._label)
 
     def insert_child(self, index, child):
+        canvas = self.canvas()
         self._subtrees.insert(index, child)
         self._add_child_widget(child)
         self._lines.append(canvas.create_line(0,0,0,0, fill='#006060'))

nltk/tree.py

@@ -1482,7 +1482,7 @@ def demo():
 
     # Demonstrate tree parsing.
     s = '(S (NP (DT the) (NN cat)) (VP (VBD ate) (NP (DT a) (NN cookie))))'
-    t = Tree.fromstring(s)
+    t = tree.Tree.fromstring(s)
     print("Convert bracketed string into tree:")
     print(t)
     print(t.__repr__())