OwPythagorasTree: Fix redraw crash when no data

Author: pavlin-policar

Orange/widgets/visualize/owpythagorastree.py

@@ -224,6 +224,8 @@ def update_size_calc(self):
         self.invalidate_tree()
 
     def redraw(self):
+        if self.data is None:
+            return
         self.tree_adapter.shuffle_children()
         self.invalidate_tree()