App crashing due to NSInvalidArgumentException but can't find it - ios

Question

My app is crashing upon running it with the following debug:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'NSConcreteMutableAttributedString initWithString:attributes:: nil value'
*** First throw call stack:
(
0 CoreFoundation 0x0000000102a83495 __exceptionPreprocess + 165
1 libobjc.A.dylib 0x00000001027e299e objc_exception_throw + 43
2 CoreFoundation 0x0000000102a832ad +[NSException raise:format:] + 205
3 Foundation 0x00000001023d993e -[NSConcreteMutableAttributedString initWithString:attributes:] + 129
4 myApp 0x0000000100006da5 -[WelcomeViewController viewDidLoad] + 373
5 UIKit 0x000000010148559e -[UIViewController loadViewIfRequired] + 562
6 UIKit 0x0000000101485777 -[UIViewController view] + 29
7 UIKit 0x00000001017902e2 -[UIClientRotationContext initWithClient:toOrientation:duration:andWindow:] + 390
8 UIKit 0x00000001013cbffa -[UIWindow _setRotatableClient:toOrientation:updateStatusBar:duration:force:isRotating:] + 1109
9 UIKit 0x00000001013cbb9f -[UIWindow _setRotatableClient:toOrientation:updateStatusBar:duration:force:] + 36
10 UIKit 0x00000001013cbaef -[UIWindow _setRotatableViewOrientation:updateStatusBar:duration:force:] + 101
11 UIKit 0x00000001013cadfe -[UIWindow _updateToInterfaceOrientation:duration:force:] + 377
12 UIKit 0x000000010148970a -[UIViewController _tryBecomeRootViewControllerInWindow:] + 147
13 UIKit 0x00000001013c5b1b -[UIWindow addRootViewControllerViewIfPossible] + 490
14 UIKit 0x00000001013c5c70 -[UIWindow _setHidden:forced:] + 282
15 UIKit 0x00000001013ceffa -[UIWindow makeKeyAndVisible] + 51
16 UIKit 0x000000010138ac98 -[UIApplication _callInitializationDelegatesForURL:payload:suspended:] + 1788
17 UIKit 0x000000010138ea0c -[UIApplication _runWithURL:payload:launchOrientation:statusBarStyle:statusBarHidden:] + 660
18 UIKit 0x000000010139fd4c -[UIApplication handleEvent:withNewEvent:] + 3189
19 UIKit 0x00000001013a0216 -[UIApplication sendEvent:] + 79
20 UIKit 0x0000000101390086 _UIApplicationHandleEvent + 578
21 GraphicsServices 0x00000001047dd71a _PurpleEventCallback + 762
22 GraphicsServices 0x00000001047dd1e1 PurpleEventCallback + 35
23 CoreFoundation 0x0000000102a05679 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 41
24 CoreFoundation 0x0000000102a0544e __CFRunLoopDoSource1 + 478
25 CoreFoundation 0x0000000102a2e903 __CFRunLoopRun + 1939
26 CoreFoundation 0x0000000102a2dd83 CFRunLoopRunSpecific + 467
27 UIKit 0x000000010138e2e1 -[UIApplication _run] + 609
28 UIKit 0x000000010138fe33 UIApplicationMain + 1010
29 myApp 0x0000000100006b03 main + 115
30 libdyld.dylib 0x00000001033c25fd start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
The problem is that I know for a fact I never used NSConcreteMutableAttributedString I have checked and double checked my code to see if maybe I am missing something but I keep coming up blank, any ideas as to what can be done to fix this problem?
Update
I added an exception breakpoint which told me that the following line was causing my app to crash:
NSMutableAttributedString *thirdGreetingAttr = [[NSMutableAttributedString alloc] initWithString:thirdGreeting attributes:secondGreetingAttrs19];
self.welcomeUserLabel.attributedText = thirdGreetingAttr;
"Ok, odd" I think to myself, this was working just fine 2 weeks ago when I last touched this project, This is the complete code block:
NSDictionary *secondGreetingAttrs19 = [NSDictionary dictionaryWithObjectsAndKeys:fitnestFontBold, NSFontAttributeName, nil];
NSString *thirdGreeting = currentUser.username;
NSMutableAttributedString *thirdGreetingAttr = [[NSMutableAttributedString alloc] initWithString:thirdGreeting attributes:secondGreetingAttrs19];
self.welcomeUserLabel.attributedText = thirdGreetingAttr;
So I go ahead and comment out the 2 lines that are causing the issue, and the app runs just fine. I un-comment the lines back to how it was originally and to my surprise build runs JUST FINE :/ what??? Anyone know what could have happened here?
UPDATE
As the chosen answer says, I realized that this error was happening because my username property was returning a null value.

Answer 1

make sure your thirdGreeting(NSString *thirdGreeting = currentUser.username; ) is not nil value.
I think the thireGreeting is nil, you should change to
NSString *thirdGreeting = (currentUser.username.length>0) ? currentUser.username.length : #"";