-[_SwiftValue unsignedIntegerValue]: unrecognized selector sent to instance 0x608000243d50 (lldb) - swift

Question

I finished migrating my application to swift3 but this section keeps yielding is causing problems.
override func viewWillAppear(_ animated: Bool)
{
super.viewWillAppear(animated)
self.navigationController?.setNavigationBarHidden(false, animated: animated)
self.navigationItem.title = artworkTitle
let title = "<center><span style=\"font-size: 17px;font-weight:lighter;font-family:Avenir-Book;\">" + artworkCaption + "</span></center>"
artworkImageView.image = UIImage(named: artworkImagePath)
artworkCaptionView.attributedText = title.html2AttStr
}
Important Console Parts
-[_SwiftValue unsignedIntegerValue]: unrecognized selector sent to instance 0x608000243d50
terminating with uncaught exception of type NSException
(lldb)
extension String
{
var html2AttStr:NSAttributedString
{
let contents: NSMutableAttributedString?
do {
let attrTextStyle = NSMutableParagraphStyle()
attrTextStyle.alignment = NSTextAlignment.center
contents = try NSMutableAttributedString(data: data(using: String.Encoding.utf8)!, options:[NSDocumentTypeDocumentAttribute:NSHTMLTextDocumentType, NSCharacterEncodingDocumentAttribute: String.Encoding.utf8], documentAttributes: nil)
} catch _ {
contents = nil
}
Console
0 CoreFoundation 0x000000010b70934b __exceptionPreprocess + 171
1 libobjc.A.dylib 0x000000010ad7521e objc_exception_throw + 48
2 CoreFoundation 0x000000010b778f34 -[NSObject(NSObject) doesNotRecognizeSelector:] + 132
3 CoreFoundation 0x000000010b68ec15 ___forwarding___ + 1013
4 CoreFoundation 0x000000010b68e798 _CF_forwarding_prep_0 + 120
5 UIFoundation 0x0000000114565953 -[NSHTMLReader _loadUsingWebKit] + 1329
6 UIFoundation 0x0000000114566f15 -[NSHTMLReader attributedString] + 22
7 UIFoundation 0x00000001144fd45c _NSReadAttributedStringFromURLOrData + 5779
8 UIFoundation 0x00000001144fbd35 -[NSAttributedString(NSAttributedStringUIFoundationAdditions) initWithData:options:documentAttributes:error:] + 115
9 pg 0x000000010a39fe1d _TTOFCSo25NSMutableAttributedStringcfzT4dataV10Foundation4Data7optionsGVs10DictionarySSP__18documentAttributesGSqGVs33AutoreleasingUnsafeMutablePointerGSqCSo12NSDictionary____S_ + 173
10 pg 0x000000010a39fba4 _TFCSo25NSMutableAttributedStringCfzT4dataV10Foundation4Data7optionsGVs10DictionarySSP__18documentAttributesGSqGVs33AutoreleasingUnsafeMutablePointerGSqCSo12NSDictionary____S_ + 100
11 pg 0x000000010a39f96a _TFE2pgSSg11html2AttStrCSo18NSAttributedString + 458
12 pg 0x000000010a3a901d _TFC2pg21ViewControllerArtwork14viewWillAppearfSbT_ + 1197
13 pg 0x000000010a3a90b1 _TToFC2pg21ViewControllerArtwork14viewWillAppearfSbT_ + 49
14 UIKit 0x000000010c642de3 -[UIViewController _setViewAppearState:isAnimating:] + 692
15 UIKit 0x000000010c6434f3 -[UIViewController __viewWillAppear:] + 147
16 UIKit 0x000000010c67e1a3 -[UINavigationController _startTransition:fromViewController:toViewController:] + 890
17 UIKit 0x000000010c67f0b9 -[UINavigationController _startDeferredTransitionIfNeeded:] + 874
18 UIKit 0x000000010c68019b -[UINavigationController __viewWillLayoutSubviews] + 58
19 UIKit 0x000000010c8771b7 -[UILayoutContainerView layoutSubviews] + 223
20 UIKit 0x000000010c560344 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 1237
21 QuartzCore 0x000000010c312cdc -[CALayer layoutSublayers] + 146
22 QuartzCore 0x000000010c3067a0 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 366
23 QuartzCore 0x000000010c30661e _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 24
24 QuartzCore 0x000000010c29462c _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 280
25 QuartzCore 0x000000010c2c1713 _ZN2CA11Transaction6commitEv + 475
26 UIKit 0x000000010c495067 _UIApplicationFlushRunLoopCATransactionIfTooLate + 206
27 UIKit 0x000000010cca4b30 __handleEventQueue + 5672
28 CoreFoundation 0x000000010b6ae311 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
29 CoreFoundation 0x000000010b69359c __CFRunLoopDoSources0 + 556
30 CoreFoundation 0x000000010b692a86 __CFRunLoopRun + 918
31 CoreFoundation 0x000000010b692494 CFRunLoopRunSpecific + 420
32 GraphicsServices 0x0000000111084a6f GSEventRunModal + 161
33 UIKit 0x000000010c49bf34 UIApplicationMain + 159
34 pg 0x000000010a3acaaf main + 111
35 libdyld.dylib 0x000000011233668d start + 1
36 ??? 0x0000000000000001 0x0 + 1
libc++abi.dylib: terminating with uncaught exception of type NSException

Answer 1

I remember reading on Apple's Swift blog that if you get unrecognized selector errors, it means that the auto-wrapping of Swift structs are causing issues. Most likely it is wrapping String.Encoding instead of converting it to an NSNumber (what the Objective-C code behind NSMutableAttributedString is expecting).
Try replacing NSCharacterEncodingDocumentAttribute: String.Encoding.utf8 with NSCharacterEncodingDocumentAttribute: String.Encoding.utf8.rawValue