There appears to be a bug in the Data module, which became evident since installing Magnolia 4.3.6 (Data module 1.4.2). In the Company (Example), when creating a new company or amending the current one:

• The top-level data item is created without a problem.
• When you select the top-level item, the selector dialog opens and then the appropriate second level dialog.
• The problem comes in when you try to save the content of the second (and third) level dialog the content is not saved and the dialog simply remains open.

I have attached two screenshots to demonstrate.