Anas mansour, this error could mean that a plugin has added some widgets on your site and these widgets are trying to call some methods. These methods can't be called because the plugin was deactivated incorrectly (via DB). Also it means that the plugin has not passed through the deactivation process and the: buttons, links etc. were not deleted. Or the error has caused by the removing plugin source code.

So, if you got these error messages:
1. You need to go to your database, via phpmyadmin, go to the ow_base_plugin table  and make sure the plugin is enabled  (isActive=1).
2. If the plugin is enabled, most likely the problem is that the plugin source code was deleted. In this case,  you need to upload the plugin source code to the ow_plugins folder. This should remove the error and allow you to access the site through the admin panel where you can uninstall this plugin correctly.

You also can use this related thread: http://www.oxwall.org/forum/topic/24428

P.S. Never delete the plugin source code or deactivate the plugin via database because it can leads to such errors. If you don't need the plugin, it is required to uninstall it through the Admin area.