Exception on config var type change

[Павел Бельтюков]

Stock Beremiz throws unhandled exception on configuration variable data type change.

Beremiz does not crash and continues to work after that.

[Andrey Skvortsov]

fix problem with clearing non-empty location field after changing type of config variable

introduced by cd9db17 ("clear variable location cell in variable panel, if variable class or type changed").

Closes #26

→ <<cset eed01e0ab32a>>