avoid crash when window is null in become_first_responder

[httnn]

apparently newer Rust versions (starting with 1.86?) have stricter debug assertions regarding unsafe code which caused a crash under debug builds because window was null in the become_first_responder method.

this fix simply checks whether window is null or not.

[micahrj]

It looks like the build is failing due to warnings in the objc crate. Those warnings have been addressed on the master branch but not on the latest version on crates.io: SSheldon/rust-objc#125

Long-term I think the best thing to do would be to switch to the objc2 crate, but my recommendation for now is to add the following to the top of lib.rs:

#![allow(unexpected_cfgs)]