Use separate kv file

I have been trying a lot to figure out how to use a separate kv file to define the UI. Below is the error traceback which I'm getting after running.

 Traceback (most recent call last):
   File "C:/Users/Pavitra/PycharmProjects/ComponentCreator/main.py", line 59, in <module>
     CreatorApp().run()
   File "C:\Python36\lib\site-packages\kivy\app.py", line 801, in run
     self.load_kv(filename=self.kv_file)
   File "C:\Python36\lib\site-packages\kivy\app.py", line 598, in load_kv
     root = Builder.load_file(rfilename)
   File "C:\Python36\lib\site-packages\kivy\lang\builder.py", line 301, in load_file
     return self.load_string(data, **kwargs)
   File "C:\Python36\lib\site-packages\kivy\lang\builder.py", line 350, in load_string
     parser = Parser(content=string, filename=fn)
   File "C:\Python36\lib\site-packages\kivy\lang\parser.py", line 392, in __init__
     self.parse(content)
   File "C:\Python36\lib\site-packages\kivy\lang\parser.py", line 501, in parse
     objects, remaining_lines = self.parse_level(0, lines)
   File "C:\Python36\lib\site-packages\kivy\lang\parser.py", line 605, in parse_level
     level + 1, lines[i:], spaces)
   File "C:\Python36\lib\site-packages\kivy\lang\parser.py", line 605, in parse_level
     level + 1, lines[i:], spaces)
   File "C:\Python36\lib\site-packages\kivy\lang\parser.py", line 605, in parse_level
     level + 1, lines[i:], spaces)
   File "C:\Python36\lib\site-packages\kivy\lang\parser.py", line 664, in parse_level
     if current_property[:3] == 'on_':
 TypeError: 'NoneType' object is not subscriptable