Feature writer no dataset specified

First point is that Feature Datasets in Geodatabases are often misunderstood or misused.   They are not “Folders” or “Containers” of data and should not be used as such.  The misunderstanding comes because ESRI used a Folder like icon for then in ArcCatalog that made users think they are functionally similar to File Folders.

Feature Datasets are instead a separate object that stores relational data about 2 or more geographic/geometric feature classes and enforces rules and restrictions on those member feature classes.  This is necessary for the most common use of Datasets for when say a Geometric Network or Utility Network or Topology object is required to be created, that are logical tables of spatial relationships between the member Feature Classes.

As such, don't use Feature Datasets to group data, and they are useless for Geodatabase Tables (non-geometry tables) given they are primarily for forms of spatial Topology table building

This may also be why the Write fails as No Geometry features will not instantiate a Feature Class and instead instantiate a Table that cannot be a member of a Feature Dataset.