I was working on k2 today. all is working as usual.
but when I uninstalled K2 and again reinstalled it then I got this error on all my category and article

Notice: Trying to get property of non-object in C:\wamp\www\shaper\libraries\joomla\factory.php on line 223
Call Stack
# Time Memory Function Location
1 0.0014 382320 {main}( ) ..\index.php:0
2 0.5772 8821416 JSite->dispatch( ) ..\index.php:42
3 0.5852 8891912 JComponentHelper::renderComponent( ) ..\application.php:197
4 0.5928 8908200 JComponentHelper::executeComponent( ) ..\helper.php:351
5 0.5978 8971800 require_once( 'C:\wamp\www\shaper\components\com_k2\k2.php' ) ..\helper.php:383
6 0.6611 9654640 JController->execute( ) ..\k2.php:52
7 0.6612 9654696 K2ControllerItemlist->display( ) ..\controller.php:760
8 0.6956 11099792 JController->display( ) ..\itemlist.php:32
9 0.7077 11564768 K2ViewItemlist->display( ) ..\controller.php:721
10 0.8414 12670560 JCacheControllerCallback->call( ) ..\view.html.php:336
11 0.8414 12671184 JCacheControllerCallback->get( ) ..\callback.php:43
12 0.8420 12714832 call_user_func_array ( ) ..\callback.php:159
13 0.8420 12714856 K2ModelItem::prepareItem( ) ..\callback.php:159
14 0.8941 12779280 JFactory::getUser( ) ..\item.php:232

( ! ) Notice: Trying to get property of non-object in C:\wamp\www\shaper\libraries\joomla\factory.php on line 223
Call Stack
# Time Memory Function Location
1 0.0014 382320 {main}( ) ..\index.php:0
2 0.5772 8821416 JSite->dispatch( ) ..\index.php:42
3 0.5852 8891912 JComponentHelper::renderComponent( ) ..\application.php:197
4 0.5928 8908200 JComponentHelper::executeComponent( ) ..\helper.php:351
5 0.5978 8971800 require_once( 'C:\wamp\www\shaper\components\com_k2\k2.php' ) ..\helper.php:383
6 0.6611 9654640 JController->execute( ) ..\k2.php:52
7 0.6612 9654696 K2ControllerItemlist->display( ) ..\controller.php:760
8 0.6956 11099792 JController->display( ) ..\itemlist.php:32
9 0.7077 11564768 K2ViewItemlist->display( ) ..\controller.php:721
10 0.8414 12670560 JCacheControllerCallback->call( ) ..\view.html.php:336
11 0.8414 12671184 JCacheControllerCallback->get( ) ..\callback.php:43
12 0.8420 12714832 call_user_func_array ( ) ..\callback.php:159
13 0.8420 12714856 K2ModelItem::prepareItem( ) ..\callback.php:159
14 0.9373 12987344 K2HelperRoute::getUserRoute( ) ..\item.php:234
15 0.9373 12987544 JFactory::getUser( ) ..\route.php:52

Hi. These are just notices and you can hide them by setting error reporting to "None". However you should not get any notices with K2. Are you using the latest versions of K2 and Joomla!?