v218 touch unresponsive and step mode hangs

[quozl]

On upgrade from v215 to v218, two problems observed by a teacher;

• it is more difficult to use touch to deploy a block to the canvas,
• step mode stops, and log contains errors.

Traceback (most recent call last):
  File "/home/olpc/Activities/TurtleBlocks.activity/TurtleArt/talogo.py", line 627, in doevalstep
    self.step.next()
  File "/home/olpc/Activities/TurtleBlocks.activity/TurtleArt/talogo.py", line 596, in _evalsym
    result = self.cfun.fcn(self, *self.arglist)
  File "/home/olpc/Activities/TurtleBlocks.activity/TurtleArt/taprimitive.py", line 325, in __call__
    new_prim.call_afterwards(*new_args, **new_kwargs)
  File "/home/olpc/Activities/TurtleBlocks.activity/TurtleArt/tabasics.py", line 1311, in after_move
    self.tw.coord_scale)
  File "/home/olpc/Activities/TurtleBlocks.activity/TurtleArt/talogo.py", line 1097, in update_label_value
    for block in self.value_blocks_to_update[name]:
AttributeError: LogoCode instance has no attribute 'value_blocks_to_update'

org.laptop.TurtleArtActivity-2.log