So I'm trying to upgrade my Umbraco project from 7.15.5 to 8.X.X. Obviously I first changed all obsolete data types. Then I got an error, where the new types (I guess in a MultiNodeTreepicker) need to be UDIs instead of IDs. To solve that problem, I followed the steps in this guide: Sniper.Umbraco. I set a breakpoint on the MultiNodeTreePickerIdToUdiMigrator to see if it works, but well it doesn't even get into the breakpoint, because this error fires up first:

Any help would be highly appreciated! Thanks in advance!

Gogo