Fix issue where latter half of slides won't load

[lanesawyer]

Describe the bug
When loading data with multiple datasets, there is a bug where the slides won't load correctly.

To Reproduce
Steps to reproduce the behavior:

1. Go to the OME-Zarr example viewer on the demo site: https://alleninstitute.github.io/vis/
2. Load the custom URL: https://d3hik2dwvr9wk1.cloudfront.net/A549/organelle_box_crop_v1.zarr/ACTB/Live/003001
3. Notice that nothing loads, although there are 38 slides
4. Navigate back one slide, everything lower than 20 loads

Expected behavior
All slides load

Screenshots

Additional context
It's something to do with the datasets and arrays. I see a zarr.get call from Zarrita failing to load, and it's complaining about an index out of bounds since 38 is too high for 19 and 10. Which is that third number in the data structure in arrays