When you open the wizard in Visual Studio 2019 Preview 1, available Controllers are not shown in the list. As a result, callback-aware extensions, which always require a separate PartialView name and Controller Action method, cannot be inserted.
This issue does not exist in Visual Studio 2019 released on April 2nd, 2019.
We always recommend using official Visual Studio builds with the latest Service Packs installed to get actual bug fixes and enchantments.
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