When importing a document containing a single associated widget annotation merged into the field dictionary the field is skipped on import and an exception is thrown.

According to the PDF Specification: Field Dictionaries: Kids:

An array of indirect references to the immediate children of this field.
In a non-terminal field, the Kids array is required to refer to field dictionaries that are immediate descendants of this field. In a terminal field, the Kids array ordinarily must refer to one or more separate widget annotations that are associated with this field. However, if there is only one associated widget annotation, and its contents have been merged into the field dictionary, Kids must be omitted.