EXC_BRREAKPOINT when trying to create DispatchSource.makeFileSystemObjectSource() to monitor changes to file

Question

I'm an experienced iOS developer but I'm new to developing for macOS. In this app, the user has chosen a file and I want to monitor that file for any changes so that my app can respond, read the new contents of the file, and act appropriately.

I found this article about watching for changes and have been following along, but I get a strange crash/break when I run the app. The code looks like this:

I don't get any error message in the console. No stack trace. I don't have any breakpoints set. If I comment out the source.setEventHandler lines I still get the same crash/break. If I comment out the let source = DispatchSource.makeFileSystemObjectSource() lines, then the code runs fine.

The url in question is a security-scoped url that was previously chosen with an NSOpenPanel() and then stored as a security-scoped bookmark.

Am I doing something wrong? I'm new to Swift 5.5 concurrency, is that causing the problem?

How can I properly watch a file for changes so that my app can automatically respond?

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Text version of code for accessibility:

override func viewDidAppear() {
    super.viewDidAppear()

    Task {
        self.configurl = await self.openfile(forkey: self.keybookmarkconfig)
                    
        if let url = self.configurl {
            
            let configfilehandle = try FileHandle(forReadingFrom: url)
            
            let source = DispatchSource.makeFileSystemObjectSource(
                fileDescriptor: configfilehandle.fileDescriptor,
                eventMask: .extend,
                queue: DispatchQueue.main
            )
            
            source.setEventHandler {
                print("config file changed")
            }
            
        }
        
    }
    
}