Fatal error when saving changes

I upgraded the plugin, activated the “No emails” checkbox, clicked on the “Save changes” button and this appeared :

Fatal error: Uncaught Exception: wp-config.php is not writable. in /wordpress-beta-tester/vendor/WPConfigTransformer.php:49 
Stack trace: 
#0 /wordpress-beta-tester/src/WPBT_Extras.php(264): WPBT_WPConfigTransformer->__construct('/wordpress-beta-..') 
#1 /wordpress-beta-tester/src/WPBT_Extras.php(231): WPBT_Extras->remove_constants(Array) 
#2 /wordpress-beta-tester/src/WPBT_Extras.php(161): WPBT_Extras->update_constants(Array, Array) 
#3 /wp-includes/class-wp-hook.php(288): WPBT_Extras->save_settings(Array) 
#4 /wp-includes/class-wp-hook.php(312): WP_Hook->apply_filters(NULL, Array) 
#5 /wp-includes/plugin.php(478): WP_Hook->do_action(Array) 
#6 /wordpress-beta-tester/src/WPBT_Settings.php(104 in /wordpress-beta-tester/vendor/WPConfigTransformer.php on line 49

After the wp-config.php was chmod-ed to writeable the error disappeared. I recommend to check for write permissions before changing the file.