Magento [SOLVED]Can not login to admin panel and website footer error 'Access to undeclared static property'

Fatal error: Access to undeclared static property: Mage::$_isDeveloperMode in app/Mage.php on line 881

Go to Mage.php line number 881 find the function.

public static function getIsDeveloperMode()
{
    return self::$_isDeveloperMode;
}

This method simply returns a static property $_isDeveloperMode of the class app\Mage.php. You can find that the definition of that static property
 in that file itself in line 117 (magento 1.9.1) like this.

Add below code of the class app\Mage.php "static private $_isDeveloperMode = false;"

Find: static private $_isDownloader = false;

    /**
     * Is developer mode flag
     *
     * @var bool
     */
ADD: static private $_isDeveloperMode = false;

I tried the solution and it work fine.

Please feel free to contact me any Magento issue at any time.

Regards
Kuldeep
kuldeep4110@gmail.com