The error indicates that the modifier has not been setup correctly. For example if you add an armature modifier to a mesh. Then go into the mesh modifier properties and delete the Object option from the armature modifier. This will make create a situation where the mesh has a modifier but it is now in an invalid state.
So basically this is a Blender error not from the addon.
Removing the armature modifier and reattaching it to the mesh should fix your issue. Otherwise you need to figure out what you changed and fix it.