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Steps : 1. Create a class diagram, add a class element to it, and name the class 2. Add a few attributes to the class and name them 3. Expand the model tree and confirm the class element exists in model tree 4. Using contextual menu on the class element, transform it to interface 5. A confirmation dialog comes up, say "NO" check for the existance of the class element in the model tree. Its disappeared.
See also issue 79467 'Incompleted "Transform to class/interface" deletes the node in the model tree'
*** This issue has been marked as a duplicate of 79467 ***