Core Data rare crash "Dispatch queue: SQLQueue 0x1..."

Question

I have an odd crash in my app that's difficult to reproduce. I did run into some threading issues before which seem to be mostly fixed now but every now and then I receive a crash log, even on my own devices.

My code fetches Librarys descending by the last time a book was added. For each of these libraries I want to see the most recently added books. The code works (except for the crash) but I can't think of a reason for the crash itself.

I apologize for the weird unwrapping at if let libraries1 = libraries0 { etc. but to me it seemed to have improved the situation which also may however only be imagination.

The last possible cause of issue that I may spot by myself is to declare libraries outside of the thread of the mainContext however I'm not sure if that can be the root of the problem since I would expect the crash to be way more present if it was a hard "don't".

Below you will see my code, the // CRASH: comment indicates the line that Xcode blames the crash on.

func readEntries( count: Int = 3, completion: @escaping ([Data], [String]) -> () ) {
    
    var libraries = [Library]()
    
    let fetchRequest = makeFetchRequest(LIBRARY)
    let sort = NSSortDescriptor(key: "lastAddedTime", ascending: false)
    fetchRequest.sortDescriptors = [sort]
    fetchRequest.fetchLimit = count
    
    mainContext.perform {
        
        do {
            let libraries0 = try self.mainContext.fetch(fetchRequest) as? [Library]
            if let libraries1 = libraries0 {

                libraries = libraries1
                
                for lib in libraries {
                    
                    if(lib.lastAddedTime > 0) {

                        let fetchRequest = self.makeFetchRequest(self.BOOK)
                        fetchRequest.fetchLimit = 5

                        let predicate = NSPredicate(format: "hostLibrary == %@", lib)
                        fetchRequest.predicate = predicate
                        fetchRequest.sortDescriptors = [NSSortDescriptor(key: "time", ascending: false)]

                        if var books = try? self.mainContext.fetch(fetchRequest) as? [Book] {  // CRASH: "Dispatch queue: SQLQueue 0x1020206e0 for DBBookModel.sqlite (0)"

                            print(books)
                        }
                        
                    }
                }
                //completion ...
            }
        } catch let error as NSError {
            print("Could not load. \(error), \(error.userInfo)")
        }
    }
}