It seems there's a bug in Optimizely CMS 12. Steps to reproduce:

• have a page type (A) that has ContentArea property
• have a block type (B) that has ContentArea property
• create instance of block B and add it to ContentArea of A page
• publish A
• now open quick edit of block B
• try to create new block into B's ContentArea by clicking Select Content -> Create New Block
• 
• error: nothing happens. When you check web developer tools there's this invalid request sent to https://localhost:5001/EPiServer/cms/Stores/contentdata/NaN

Tested on EpiFoundation project https://github.com/episerver/Foundation , here's the list of packages versions: https://github.com/episerver/Foundation/blob/453664fc2bb3d915f55e270cd34135e814fdf627/src/Foundation/Foundation.csproj