`[NSString UTF8String]` sometimes returns null (it's documented as
such), and when it does, zed crashes in `window::insert_text`. I'm
running into this sometimes when using alt-d to delete forward. It
usually only happens with multiple cursors, but sometimes with a single
cursor. It *might* only happen when using the "Unicode Hex Input"
keyboard 'Input Source' (which I started using to avoid entering weird
characters in zed when using emacs meta keybindings that I haven't
defined in zed).

When using the US English input source, alt-d always results in a call
to `insert_text`. When using the Unicode Hex Input source it usually
doesn't, but when it does `text.UTF8String()` returns null. `text` isn't
null. `[text length]` returns 1. `[text characterAtIndex: 0]` seems to
always return `56797` (an undefined utf-16 codepoint).

Release Notes:

- Fixed crash on mac when deleting with alt-d