bug#28501: 26.0.60; flyspell crashes when a dictionary is changed

[Konstantin Reich]

Hey, i often write in two languages, so i change dictionaries in flyspell. In 
new emacs, when i change a dictionary i get the following error:

Error in post-command-hook (flyspell-post-command-hook): (wrong-type-argument 
stringp nil). 

To reproduce error one can eval the following code in emacs -Q:

(require 'ispell)
(require 'flyspell)
(flyspell-mode)
(ispell-change-dictionary "english")

The last line can have any dictionary, which is installed in the system and not 
the default.

I do not have such problem in emacs 25.2