Definitely an error with Zygor. According to their forums, they have had access to the Legion beta for some time, and are working on a compatible version, which will launch at or close to Legion's launch. The Legion version of Zygor will be available for
pre-order soon(tm).
As for your error, the first four lines clearly mention
ZygorGuidesViewer.
Lua Code:
Message: Interface\AddOns\ZygorGuidesViewer\Pointer.lua:4093: attempt to perform arithmetic on field 'width' (a nil value)