我的应用程序在使用以下调试运行时崩溃：
*** 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
问题在于我知道一个我从未使用的事实NSConcreteMutableAttributedString我已经检查并仔细检查了我的代码，看看我是否遗漏了一些东西，但我一直都是空白，任何关于什么都可以的想法完成修复这个问题？
更新
我添加了一个异常断点，告诉我以下行导致我的应用程序崩溃：
NSMutableAttributedString *thirdGreetingAttr = [[NSMutableAttributedString alloc] initWithString:thirdGreeting attributes:secondGreetingAttrs19];
self.welcomeUserLabel.attributedText = thirdGreetingAttr;
“好的，奇怪的”我觉得，自从我上次接触这个项目后，这个工作正常，这是完整的代码块：
NSDictionary *secondGreetingAttrs19 = [NSDictionary dictionaryWithObjectsAndKeys:fitnestFontBold, NSFontAttributeName, nil];
NSString *thirdGreeting = currentUser.username;
NSMutableAttributedString *thirdGreetingAttr = [[NSMutableAttributedString alloc] initWithString:thirdGreeting attributes:secondGreetingAttrs19];
self.welcomeUserLabel.attributedText = thirdGreetingAttr;
所以我继续说明导致问题的2行，并且app运行得很好。我取消评论线条回到最初的原因，并且令我惊讶的是构建运行JUST FINE：/ what ???有人知道这里会发生什么吗？
更新
正如所选答案所说，我意识到发生了此错误，因为我的username属性返回了null值。
答案 0 :(得分：6)
确保您的thirdGreeting（NSString * thirdGreeting = currentUser.username;）不是nil值。 我认为thireGreeting是零，你应该改为
NSString *thirdGreeting = (currentUser.username.length>0) ? currentUser.username.length : @"";