Fix undefined access on not-existing segmentation layer

[philippotto]

URL of deployed dev instance (used for testing):

• https://fixundefinedmesh.webknossos.xyz

Steps to test:

• open a dataset without segmentation layer in view mode and switch to the Meshes tab (within a hybrid annotation, an segmentation layer would always exist)
• ensure that the console doesn't print Uncaught (in promise) TypeError: UnhandledRejection: "TypeError: Cannot read property 'fallbackLayer' of null"

Issues:

• fixes airbrake reports I saw with Uncaught (in promise) TypeError: UnhandledRejection: "TypeError: Cannot read property 'fallbackLayer' of null"

---

• Updated (unreleased) changelog
• Updated (unreleased) migration guide if applicable
• Updated documentation if applicable
• Adapted wk-connect if datastore API changes
• Needs datastore update after deployment
• Ready for review

[philippotto]

Took me a while to figure out why the errors regarding the Props were not caught by flow. It turns out, flow simply assumed the props to be any. The reason for this is that the extraction of the return type for these functions here didn't work. After a bit of digging I found out that the * as a return type helps 🤦

Here's the comment: facebook/flow#7071 (comment)
The issue was opened 2.5 years ago. Yet another reason to ditch flow?

I also looked for ExtractReturn and there is only one other usage which doesn't seem to suffer from the same bug. So, hopefully no other bad suprises.

[daniel-wer]

Nice find and fix! Thank you for spending the extra time to get to the root of why flow didn't catch this 🎉