Symphony Notice: Undefined variable: useragent

[bauhouse]

Affected Symphony version(s) : 3.0.0
PHP version(s) : 7.3.7
MySQL version(s): 5.7.26 Homebrew
OS(es) : macOS Catalina 10.15.3

The Preferences page throws the following error when the Maintenance Mode extension is enabled for Symphony 3.0:

An error occurred in /Users/stephen/Sites/symphonycms/extensions/maintenance_mode/extension.driver.php around line 93

88         $label = Widget::Label(__('Useragent Whitelist'));
89         $whitelist = json_decode(Symphony::Configuration()->get('useragent_whitelist', 'maintenance_mode'));
90         if (is_array($whitelist) && !empty($whitelist)) {
91             $useragent = implode("\r\n",$whitelist);
92         }
93         $label->appendChild(Widget::Textarea('settings[maintenance_mode][useragent_whitelist]', 5, 50, General::sanitize($useragent)));
94         $group->appendChild($label);
95 
96         // Append help
97         $group->appendChild(new XMLElement('p', __('Any useragent that listed above will be granted access. This eliminates the need to allow a

The error goes away by modifying the conditional.

// Useragent White list
$label = Widget::Label(__('Useragent Whitelist'));
$whitelist = json_decode(Symphony::Configuration()->get('useragent_whitelist', 'maintenance_mode'));
if (is_array($whitelist) && !empty($whitelist)) {
    $useragent = implode("\r\n",$whitelist);
    $label->appendChild(Widget::Textarea('settings[maintenance_mode][useragent_whitelist]', 5, 50, General::sanitize($useragent)));
    $group->appendChild($label);
}