Hi everyone!
I got the same issue and I've found resolution. I opened .dfm and remove the only one string from tree view block "Version ='xxx'". After that form opened in IDE and tree view upgraded successfully.
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This solution worked for me! I had been getting this error after opening forms with TAdvTreeViews on them (or descendants), and ignoring the Delphi IDE errors:
Error creating form: No mapping for the Unicode character exists in the target multi-byte code page.
So, like Otstavnov said, I opened those particular .dfm's, searched for "Version =", removed those lines, saved them, and they were able to open in Delphi (although they still had the initial errors and warnings with component property values).