For migrating the SharePoint 2010 to SharePoint 2013, After upgraded the solution from Visual studio 2012 frame work 3.5 to Visual studio 2013 .Net frame work 4.5, After getting the error message in UserControl(.ascx) On placing the cursor over the Toolbar in ascx showing the error like Element ToolBar is not a Known Element. This can occur there is a compilation error in the web site, or the web.config file is missing. And by clicking on Design page getting the rendering error message. In visual studio 2012 .Net frame work 3.5 we are not getting the Rendering error message, but getting the render error message in Visual studion 2013 .Net frame work 4.5 in the UserControl.
Could you please let me know how to resolve the issue.