Please have a try with following steps to copy and paste the TopDesign.cysch from one to another project:
1. In workspace browser, go to tab-Component,
2. Right click TopDesign, choose Export component->Create New Archive...,
3. Open a new design project and go to its tab-Component.
4. Right click Project ‘[Your Project Name]’ [Part Number], choose Import component->Import from Archive,
5. Find the Archive created in step#2 and click OK.
6. Set it as the Top Component.
Kindly try this to check if it can work.
thank you for your answer. This method also don't work. The Top Design archive is very small. When I importing it in a new clean design an error message is generated (see attachment).
Heater 16C1k-32P.cycomp.zip 1,000 bytes