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Create a new XML file from the "XML with DTD" template. Open it in the tree view editor and select the doctype node immediately below the comment. Delete the "<DEFAULT>" entry from the system ID (or replace it with some junk text). Immediately you tab out of the field, the tree changes to a single "Parse fatal error..." node, which is to be expected. However, there's then no way to change the invalid entry, and the Undo edit menu item is disabled... The only way to fix it is to reopen it the text editor and change it there. Since this situation could occur simply because of a mistyped URL or filename, it would be better if there was a way to correct it straight away within the tree view.
I do not know, how to implement UnDo in Customizer component -- ui master could add comment. Undo/redo action on tree could help (issue 15791).
Let these are considered while planning next release.
Target milestone was changed from not determined to TBD
How to address undo in bean customizers?
Fixed long time ago
Verified - fixed long time ago