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kivy simple app elements disappears when window resized by mouse

I run a test kivy app as in the tutorial. However, when I resize the window by mouse, all elements disappear and a black window is left.
Please tell me why and help me solve it.

from kivy.app import App
#kivy.require('1.9.1')

from kivy.uix.label import Label
from kivy.uix.gridlayout import GridLayout
from kivy.uix.textinput import TextInput

class LoginScreen(GridLayout):
    def __init__(self, **kwargs):
        super(LoginScreen, self).__init__(**kwargs)
        self.cols = 2
        self.add_widget(Label(text="Username:"))
        self.username = TextInput(multiline=False)
        self.add_widget(self.username)

class SimpleKivy(App):
    def build(self):
        return LoginScreen()

if __name__ == "__main__":
    SimpleKivy().run()

This is the corresponding console in case related:
/Library/Frameworks/Python.framework/Versions/3.5/bin/python3.5 /Users/vucheqh/PycharmProjects/mobile/KivyTest.py
[INFO              ] [Logger      ] Record log in /Users/vucheqh/.kivy/logs/kivy_16-08-26_12.txt
[INFO              ] [Kivy        ] v1.9.1
[INFO              ] [Python      ] v3.5.2 (v3.5.2:4def2a2901a5, Jun 26 2016, 10:47:25)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)]
[INFO              ] [Factory     ] 179 symbols loaded
[INFO              ] [Image       ] Providers: img_tex, img_imageio, img_dds, img_gif, img_pygame, img_pil (img_ffpyplayer ignored)
[INFO              ] [Text        ] Provider: pygame
[INFO              ] [Window      ] Provider: pygame
[ERROR             ] [WinPygame   ] unable to set icon
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/kivy/core/window/window_pygame.py", line 215, in set_icon
    self._set_icon_standard(filename)
  File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/kivy/core/window/window_pygame.py", line 227, in _set_icon_standard
    im = pygame.image.load(filename)
pygame.error: Failed loading libpng.dylib: dlopen(libpng.dylib, 2): image not found
[INFO              ] [GL          ] OpenGL version <b'2.1 INTEL-10.14.73'>
[INFO              ] [GL          ] OpenGL vendor <b'Intel Inc.'>
[INFO              ] [GL          ] OpenGL renderer <b'Intel HD Graphics 5000 OpenGL Engine'>
[INFO              ] [GL          ] OpenGL parsed version: 2, 1
[INFO              ] [GL          ] Shading version <b'1.20'>
[INFO              ] [GL          ] Texture max size <16384>
[INFO              ] [GL          ] Texture max units <16>
[INFO              ] [Window      ] virtual keyboard not allowed, single mode, not docked
[INFO              ] [OSC         ] using <multiprocessing> for socket
[INFO              ] [Base        ] Start application main loop
[INFO              ] [GL          ] NPOT texture support is available
[ERROR             ] [WinPygame   ] unable to set icon
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/kivy/core/window/window_pygame.py", line 215, in set_icon
    self._set_icon_standard(filename)
  File "/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/kivy/core/window/window_pygame.py", line 227, in _set_icon_standard
    im = pygame.image.load(filename)
pygame.error: Failed loading libpng.dylib: dlopen(libpng.dylib, 2): image not found
[INFO              ] [Context     ] Reloading graphics data...
[INFO              ] [Context     ] Reloading done in 0.0291s
Exception ignored in: 'kivy.graphics.context.Context.dealloc_texture'
OverflowError: signed integer is greater than maximum