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
