Add checking for existence of fetchMoreQuery in store

[conrad-vanl]

Fixes #3345

We are noticing that there are scenarios where this.store[fetchMoreForQueryId] is undefined, which throws an error and causes an RSOD in react-native. I am having issue reliably recreating this issue, thus I did not add test coverage for this yet - but happy to do so if we feel it's needed for this case.

I believe the logic added in this PR results in safer code, but I'm not sure if the fact that store not containing fetchMoreForQueryId points to another issue elsewhere. It seems logical to include the conditional check that's added in this PR, especially since we check for this.store[queryId] just a few lines above, but wanted to call that out.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• If this was a change that affects the external API used in GitHunt-React, update GitHunt-React and post a link to the PR in the discussion.
• Fix current tests
• Add changelog

[apollo-cla]

	Warnings
⚠️	There are library changes, but not tests. That's OK as long as you're refactoring existing code

Generated by 🚫 dangerJS

[rtymchyk]

It does seem to point to a larger issue, but given that I have no way of even catching this error, it would be good to have this guard in place. It's by far the most frequent Apollo error we face in our app.

[Jarred-Sumner]

Any update on this? This bug causes frequent crashes in React Native apps when infinite scroll is used on FlatList/SectionList and that FlatList/SectionList gets unmounted

[conrad-vanl]

As far as I can tell this PR fixes the error and crash on react native, just needs some 👀 from a maintainer

[rtymchyk]

@helfer maybe some 👀 on this? It's just a 2 liner guard :) Happy to help push this forward where needed

[hwillson]

@rtymchyk I'll have this reviewed today. We should be able to get this in place for our next release (May 29th).

[rtymchyk]

Thank you very much!

[hwillson]

Thanks for this PR @conrad-vanl! I've added a few tests to verify things, and everything looks great. We should be all set to get this merged. Thanks again!