This has nothing to do with Unity. The error came from Spine editor.
Will let @Nate know.

Do you have a spine.log file to show? (here are instructions on how to find the log file)
Is there anything you can say about your images? Are they really big? Are some of them empty? Is there a lot of them?

1000k px :o

23 Feb 2016, 13:05

also, I think I may have seen this before, or one of the artists has. I think, maybe, there is a missing picture on the skeleton. check all of your slots/images. If you cant find it, try exporting the skeleton an re-importing it. I cant quite remember, it was like a year ago :/

Have you also tried increasing the max width and max height to 2048 instead of 1024? I suspect your stick_3 and stick_4 may be too large if it has the additional padding.

Also, avoid using filename_#.png naming scheme. Spine atlases attach a special meaning to the _# at the end and you may end up with problems when you import them in your game engine. -# is fine. Even spaces is fine.

I'm not sure if this is why you're getting the error. Probably not.

But it doesn't crash if it is too large...it gives you an error.

we sometimes use #, there isn't any obvious problems.¯_(ツ)/¯

Really? I remember the libGDX texture packer used to assume they were sequences and just broke attachment paths 'cause it chopped off the _# end and assigned an index value.
Maybe Nate disabled that.

Huh. Okay, good to know!

There probably should have been an warning message instead of a NullPointerException either way. Already showed Nate the thing.

I believe this is fixed in v3.

The indices feature (_xxx) is disabled by default.