Enable App Extension Safe use of API's

[OskarGroth]

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• I've searched for any related issues and avoided creating a duplicate issue.

Description

If a target (in our case, a general UI framework we use internally) uses the Require Only App-Extension-Safe API flag, it is unable to include SlackTextViewController because it refuses to build since it uses UIApplication sharedApplication in various places.

Reproducible in:

• This is reproducible in the sample project.

Steps to reproduce:

1. Toggle Require Only App-Extension-Safe API to Yes.
2. Try building

Expected result:

Project succeeds to build

Actual result:

Project fails to build

Attachments:

e.g. Logs, screenshots, screencast, sample project, funny gif, etc.