Ok, I made minimal edits to your shipping MDI sample to show the error.
Attached please find a
TabbedMDICSSample_405.zip
which demonstrates the issue.
Upon starting the app you''ll see a "Syncfusion MDI Windows" menu which lists the open MDI children (there should be 4 to start with). Then, right-click on a tab and choose "Custom Item: Insert new doc" -- this will create a new MDI child. Note at this point that the Syncfusion MDI windows menu still functions properly. Now, repeat the Insert New Doc until you have 10 MDI children. At this point, try the SyncFusion MDI windows menu item -- it will just hang.