Operating system: Windows 7 Pro 64Bit
Graphics card: NVidia Quadro 5000
Blender 2.8 Beta
Build Date: 2019-03-31
Short description of error
FBX Exporter cannot save a bone rig without adding an empty object on top of the hierarchy. Sometimes it might be desired to have a bone hierarchy resting on the highest level.
For example: This creates compatibility issues with at least Unity which has its own bug with transferring Animation Data from a Rig with null as parent.
There might be other cases as well where a bonestructure being located on the root level instead of a parent is preferred or necessary.
Exact steps for others to reproduce the error
Create any bone rig hierarchy and export the rig only with either 'Null', 'Root' or 'limb Node' (doesn't matter) then open the FBX file in another application like Maya.