One of the quirks of working with extensions is that you cannot run an object from the object designer. This is true for V1 and V2.
With V2 you can start an object (page) after deploy but this only works once and only in the WebClient.
If you just quickly want to check our a page or codeunit in the Windows client you can write a codeunit against an object that does not exist.
An example is the TowersOfHanoi app that Microsoft ships as example. This does not have a page extension to execute itself.
Works all the time.
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