We found the possible issue:
If you have a solution folder in your solution, it will throw the exception you get, because solution folders doesn't have FullName. In the currently released version, we've made a small fix for this problem and now it will work if you have solution folder, but if you have project in this folder, it will skip them and you may not be able to add widget+designer there.
If you still reproduce the problem, please report it and we'll continue with the investigation.
the Sitefinity team
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