Diagnosis
The tab title that you deleted was associated with a subscreeen areathat had been created to display the tab page.

System Response
The associated subscreen area has also been deleted.

Procedure
If you want to retain the subscreen area, choose the "Undo" function torestore the tab title. Then move the subscreen area out of the tab pagebefore deleting the tab title again.
Note: If there is not enough room on the screen to move the subscreenarea out of the tabstrip control, you can copy it to the clipboardusing "Cut", then copy it back onto the screen using "Paste".