This means that the controller gets a reference to an Ext.tab.Panel class that contains an Ext.Toolbar with an Ext.Button in it with id mybutton. This is much better than just using id because it is more specific considering that you specified the exact path to the component.
This problem occurs mainly when jar files that you added under the Projects tab do not exist. I tend to focus my imports of jar files under the Libraries tab, hence I believe the Projects tab is often times ignored.
Right click project name, go to properties, then choose Java Build Path.
Simply go to the Projects tab and remove the jar files that were not found by your Eclipse IDE.