Crash in toolbar title code on Big Sur

Just in case it's affecting anybody else, I have filed a feedback FB7834896 detailing the fact that Big Sur crashes in AppKit's toolbar-based title view code, when I change the fileURL of my document. This is the head of a typical crash log:

Code Block Application Specific Backtrace 1:
0 CoreFoundation 0x00007fff2a9e66a3 __exceptionPreprocess + 242
1 libobjc.A.dylib 0x00007fff70324837 objc_exception_throw + 48
2 CoreFoundation 0x00007fff2aa0f34e +[NSException raise:format:arguments:] + 88
3 Foundation 0x00007fff2d545519 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 191
4 AppKit 0x00007fff277e5baf -[NSStackView insertArrangedSubview:atIndex:] + 924
5 AppKit 0x00007fff27fa3f0d -[NSToolbarPrimaryTitleContainerView _configureWithRepresentedURL:] + 97
6 AppKit 0x00007fff27fa3d68 -[NSToolbarPrimaryTitleContainerView updateTitleProperties:] + 217
7 AppKit 0x00007fff27fa5ac8 -[NSToolbarTitleView _updateTitleProperties:] + 1103


Crash in toolbar title code on Big Sur
 
 
Q