Open Radar

 
Summary:
Safari Technology Preview crashes for me since Release 37 because you've changed -[BrowserWindowController _updateKeyboardLoopIfReady] to query the BrowserWindowController's _toolbarController's lastVisibleItem's view. This will crash if the item is a NSFlexibleSpaceItem, since its view will return nil; this causes -[NSArray addObject:atIndex:] to throw an exception when this nil view is added. See (second) attached screenshot for disassembly of Safari Technology Preview.app/Contents/Frameworks/Safari.framework/Safari and analysis of the issue (the first screenshot is the original Safari.framework from /System/Library/Frameworks, which doesn't have this issue).

Steps to Reproduce:
1. Quit Safari Technology Preview, if it's running.
2. Open ~/Library/Preferences/com.apple.SafariTechnologyPreview.plist.
3. In "NSToolbar Configuration BrowserToolbarIdentifier-v2/TB Item Identifiers" add "NSFlexibleSpaceItem" to the end of the array.
4. Relaunch Safari Technology Preview.
5. Crash.

Expected Results:
No crash, Safari Technology Preview just launches with the funky toolbar layout.

Observed Results:
Crash because NSFlexibleSpaceItem's view is nil.

Version:
Release 38 (Safari 11.1, WebKit 13605.1.3.1)

Notes:
Continuation from rdar://problem/33815640, which was not resolved even though it was marked that it was.

Workaround:
For end users: Remove "NSFlexibleSpaceItem" from the end of the preferences array mentioned above.
For Apple: Check for "NSFlexibleSpaceItem" before adding its view to an array.

Configuration:
MacBook Pro (Retina, 13-inch, Early 2015) running macOS High Sierra 10.13 Beta (17A358a)