Letter widget doesn't show up

biscuit.kv

#:kivy 1.1.0

<Tile>:
    size_hint: (None, None)

    canvas.before:
        PushMatrix
        Translate:
            xy: (self.x, self.y)
        Scale:
            scale: self.scale

    canvas:
        Color:
            rgb: tuple(self.fillColor)
        Mesh:
            vertices: self.vertices
            indices: [0, 1, 2, 3, 4, 5]
            mode: 'triangle_fan'

    canvas.after:
        PopMatrix

<Letter>:
    canvas.before:
        PushMatrix

    canvas:
        Color:
            rgb: 0, 1, 0
        Rectangle:
            pos: self.pos[0], self.pos[1]
            size: 55, 55

    canvas.after:
        PopMatrix

main.py

"""
"""
from __future__ import print_function

import os
os.environ['KIVY_TEXT'] = 'pygame'
os.environ['KIVY_IMAGE'] = 'pygame'

import kivy
kivy.require("1.1.0")

from kivy.animation import Animation
from kivy.uix.anchorlayout import AnchorLayout
from kivy.uix.label import Label
from kivy.uix.relativelayout import RelativeLayout
from kivy.uix.widget import Widget
from kivy.app import App
from kivy.logger import Logger
from kivy.properties import ReferenceListProperty, NumericProperty
from math import sin, pi

import itertools
import random

SIN60 = sin(pi / 3.0)
SLOPE = (2.0 / 3.0) * SIN60
SCALE = 20.0
THICKNESS = 2
HEX_VERTICES = ([SIN60, 0], [2 * SIN60, 0.5],
                [2 * SIN60, 1.5], [SIN60, 2.0],
                [0, 1.5], [0, 0.5])


class Letter(Label):
    def __init__(self, **kw):
        if not len(kw["text"]) == 1:
            raise ValueError("Letter label must be length 1")
        kw["size_hint"] = (None, None)
        super(Letter, self).__init__(**kw)

    def on_touch_down(self, touch):
        return False


class Player(object):
    def __init__(self, pos_x, pos_y, **kw):
        super(Player, self).__init__()
        self.widget = Letter(text="@", **kw)
        self.x, self.y = pos_x, pos_y


class Tile(AnchorLayout):
    r = NumericProperty(1.0)
    g = NumericProperty(1.0)
    b = NumericProperty(1.0)
    fillColor = ReferenceListProperty(r, g, b)

    def __init__(self, hex_x, hex_y, scale=SCALE):
        self.hex_x, self.hex_y = hex_x, hex_y
        self.scale = scale
        x_pos = self.scale * (hex_x * 2.0 + (hex_y % 2)) * SIN60
        y_pos = self.scale * 1.5 * hex_y
        xyuv = [v + [0, 0] for v in HEX_VERTICES]
        self.vertices = list(itertools.chain(*xyuv))
        super(Tile, self).__init__(pos=(x_pos, y_pos),
                                   size=(2 * SIN60 * self.scale,
                                         2.0 * self.scale))

    def on_touch_down(self, touch):
        if not self.collide_point(*touch.pos):
            return False
        self.g, self.b = 0.0, 0.0
        Animation(r=0.0).start(self)
        Animation(b=1.0).start(self)
        print("Touch widget at ({0}, {1})".format(self.hex_x, self.hex_y))
        print("Color of touched widget is", self.fillColor)
        return True

    def on_touch_move(self, touch):
        if self.collide_point(*touch.pos):
            return self.on_touch_down(touch)
        return False

    def collide_point(self, *point):
        if not super(Tile, self).collide_point(*point):
            return False
        rel_x, rel_y = self.to_local(*point, relative=True)
        dy = SLOPE * rel_x
        if rel_x <= SIN60 * self.scale:
            y_min, y_max = (self.scale * 0.5) - dy, (self.scale * 1.5) + dy
        else:
            y_min, y_max = dy - (self.scale * 0.5), (self.scale * 2.5) - dy
        return (y_min < rel_y < y_max)


class Biscuit(App):
    def build(self):
        root = RelativeLayout(pos=(SCALE / 2, SCALE / 2))
        widgetArray = []
        for x in range(0, 22):
            widgetArray.append([])
            for y in range(0, 19):
                widgetArray[x].append(Tile(x, y))
                root.add_widget(widgetArray[x][y])
        player_x, player_y = random.randrange(0, 22), random.randrange(0, 19)
        player_tile = widgetArray[player_x][player_y]
        player = Player(player_x, player_y,
                        anchor_x="center", anchor_y="center",
                        text_size=(None, None))
        player_tile.add_widget(player.widget)
        player_tile.r = 0.0
        print("Player location: ({0}, {1})".format(player_x, player_y))
        print("Player position: ({0}, {1})".format(player.widget.x,
                                                   player.widget.y))
        return root

if __name__ == '__main__':
    Biscuit().run()