uncaught exemption when writing to a static dictionary in an HTTP request - swift

Question

I'm getting an Uncaught exemption within a http closure related to a dictionary stating there's an uncaught exemption. When I set a breakpoint exemptions, it points to a dictionary. The dictionary in question is declared in a struct as a static var and has multiple values already in it so how can this be happening? Here's the http request.
session.dataTask(with: request){ (data, response, error) in
if let data = data,
let tile = UIImage(data: data),
let documentsURL = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first{
let fileName = Date().timeIntervalSince1970
let filePath = documentsURL.appendingPathComponent(String(describing: fileName))
Maps.tileCachePath[url] = fileName //<- this is where the exception happens
//make sure there is no old file and if so delete it
if FileManager.default.fileExists(atPath: filePath.path){
do {
try FileManager.default.removeItem(at: filePath)
} catch{
print("error deleting old tile")
}
}
//now write the new file
FileManager.default.createFile(atPath: filePath.path, contents: data, attributes: nil)
print(filePath.path)
//return
result(tile, error)
} else {
result(nil, error)
}
}.resume()

Answer 1

It's a typo
Replace
Maps.tileCachePath[url] = fileName
with
Maps.tileCachePath[url] = filePath
Basically Date().timeIntervalSince1970 as a filename is a very bad idea. The number contains fractional seconds which are treated as a file extension.
Use a more reliable file name like a formatted date or at least remove the fractional seconds and add a real file extension.

Answer 2

Date().timeIntervalSince1970
is a double, you might need a string value there.