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Close SQL Developer. This is important. If you modify the scheme file while SQL Developer is open, your changes won't be saved.
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Locate file
dtcache.xml
in the SQL Developer's settings directory (On windows it is located in directoryC:\Users\yourUser\AppData\Roaming\SQL Developer\systemxx.xx.xx.xx.xx\o.ide.xx.x.x.x.xx.xxxxxx.xxxx
) and replace it.