Hi there,
Could you please put in a support ticket with a link to your .Spine file as well as any images which go with this spine file as well as a copy of your project which generates this error. We've improved this error message already internally and the cause of this is probably that your Atlas file to go with your .json is corrupted and is missing information which previously never threw an error but it would be useful to have the above files to help us see if we can figure out the cause of this and I'm also interested in whether this Spine worked in the previous version (2.0.5)?