Verify that SudokuView object exists before calling its methods

Before this, when changing options (highlighter and warnings) from the
menu during the 'Select Difficulty' screen the following error messages
were shown:
** (gnome-sudoku:11303): CRITICAL **: sudoku_view_set_highlighter: assertion 'self != NULL' failed
and similar for the 'warnings' option:
** (gnome-sudoku:11303): CRITICAL **: sudoku_view_set_show_warnings: assertion 'self != NULL' failed
(even though the option changes would be applied when the new game started).

https://bugzilla.gnome.org/show_bug.cgi?id=740114
Fix nullability of game and view objects