On 3.5 the issue is gone, but on 3.4 and 2.7 following traceback are generated:

$ ./python -m idlelib.idle Lib/decimal.py
Traceback (most recent call last):
  File "/home/serhiy/py/cpython-3.4/Lib/runpy.py", line 170, in _run_module_as_main
    "__main__", mod_spec)
  File "/home/serhiy/py/cpython-3.4/Lib/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/serhiy/py/cpython-3.4/Lib/idlelib/idle.py", line 11, in <module>
    idlelib.PyShell.main()
  File "/home/serhiy/py/cpython-3.4/Lib/idlelib/PyShell.py", line 1562, in main
    if flist.open(filename) is None:
  File "/home/serhiy/py/cpython-3.4/Lib/idlelib/FileList.py", line 36, in open
    edit = self.EditorWindow(self, filename, key)
  File "/home/serhiy/py/cpython-3.4/Lib/idlelib/PyShell.py", line 141, in __init__
    self.color_breakpoint_text()
  File "/home/serhiy/py/cpython-3.4/Lib/idlelib/PyShell.py", line 159, in color_breakpoint_text
    self.text.tag_config('BREAK', cfg)
AttributeError: 'NoneType' object has no attribute 'tag_config'