Here is a sample application that has the problem. It appears that the issue is easy to reproduce by changing something via the designer in frmMain (like slightly resizing the large close button larger or smaller. Or adding a label or something else in the same DocumentWindow area around the large Close button). 

Here is another issue that might be related with the Designer error I'm pointing out. Using my example Test project, If you add another "Tabbed Document" to the RadDock and then launch the application and go back to the designer and select a tab that is not currently selected in the designer. You will see the errors in the attached screen shots. This causes you to have to close out the designer window and reopen to clear the error.