Xcode 9 (Objective-C): Loading Custom XIB causes errors NSUnknownKeyException this class is not key value coding-compliant for the key

Question

I've been doing .net core stuff lately and I have started again with Xcode (and rusty) this time with Xcode 9. I created a simple Objective-C project with the following steps (which I did before on older versions of Xcode:

1. Deleted the default storyboard
2. Deleted the launchscreen
3. Modified the plist file for steps 1 and 2 above
4. Created a new "Cocoa Touch Class" with subclass of UIViewController and XIB (named vcMain)

5. Modified the appdelegate.m

   - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
       self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
       self.window.backgroundColor = [UIColor whiteColor];
   
   
       UIViewController *mainView = [[UIViewController alloc] initWithNibName:@"vcMain" bundle:nil];
   
       self.window.rootViewController = mainView;
       [self.window makeKeyAndVisible];
       return YES;
   }
   

6. Modified the vcMain.h

7. Modified the vcMain.m

8. When I run it I get the error below (which I didn't get prior to linking the outlets and received actions in the XIB).

Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[ setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key btnClient.'
*** First throw call stack:
(
    0   CoreFoundation                      0x000000010475f12b __exceptionPreprocess + 171
    1   libobjc.A.dylib                     0x0000000108cfaf41 objc_exception_throw + 48
    2   CoreFoundation                      0x000000010475f079 -[NSException raise] + 9
    3   Foundation                          0x00000001053b5a63 -[NSObject(NSKeyValueCoding) setValue:forKey:] + 292
    4   UIKit                               0x0000000106cf5117 -[UIViewController setValue:forKey:] + 87
    5   UIKit                               0x0000000106fe6c2d -[UIRuntimeOutletConnection connect] + 109
    6   CoreFoundation                      0x00000001047023cd -[NSArray makeObjectsPerformSelector:] + 317
    7   UIKit                               0x0000000106fe55e3 -[UINib instantiateWithOwner:options:] + 1856
    8   UIKit                               0x0000000106cfc3ff -[UIViewController _loadViewFromNibNamed:bundle:] + 383
    9   UIKit                               0x0000000106cfcd2b -[UIViewController loadView] + 177
    10  UIKit                               0x0000000106cfd05c -[UIViewController loadViewIfRequired] + 195
    11  UIKit                               0x0000000106cfd8b9 -[UIViewController view] + 27
    12  UIKit                               0x0000000106bc87cf -[UIWindow addRootViewControllerViewIfPossible] + 122
    13  UIKit                               0x0000000106bc8ed7 -[UIWindow _setHidden:forced:] + 294
    14  UIKit                               0x0000000106bdbe54 -[UIWindow makeKeyAndVisible] + 42
    15  [app name]                        0x00000001040b7c8f -[AppDelegate application:didFinishLaunchingWithOptions:] + 639
    16  UIKit                               0x0000000106b4cbca -[UIApplication _handleDelegateCallbacksWithOptions:isSuspended:restoreState:] + 299
    17  UIKit                               0x0000000106b4e648 -[UIApplication _callInitializationDelegatesForMainScene:transitionContext:] + 4113
    18  UIKit                               0x0000000106b53aeb -[UIApplication _runWithMainScene:transitionContext:completion:] + 1720
    19  UIKit                               0x0000000106f1d6f8 __111-[__UICanvasLifecycleMonitor_Compatability _scheduleFirstCommitForScene:transition:firstActivation:completion:]_block_invoke + 924
    20  UIKit                               0x00000001072f34c8 +[_UICanvas _enqueuePostSettingUpdateTransactionBlock:] + 153
    21  UIKit                               0x0000000106f1d2f1 -[__UICanvasLifecycleMonitor_Compatability _scheduleFirstCommitForScene:transition:firstActivation:completion:] + 249
    22  UIKit                               0x0000000106f1db6b -[__UICanvasLifecycleMonitor_Compatability activateEventsOnly:withContext:completion:] + 696
    23  UIKit                               0x000000010789ba69 __82-[_UIApplicationCanvas _transitionLifecycleStateWithTransitionContext:completion:]_block_invoke + 262
    24  UIKit                               0x000000010789b922 -[_UIApplicationCanvas _transitionLifecycleStateWithTransitionContext:completion:] + 444
    25  UIKit                               0x00000001075789c8 __125-[_UICanvasLifecycleSettingsDiffAction performActionsForCanvas:withUpdatedScene:settingsDiff:fromSettings:transitionContext:]_block_invoke + 221
    26  UIKit                               0x0000000107777b06 _performActionsWithDelayForTransitionContext + 100
    27  UIKit                               0x000000010757888b -[_UICanvasLifecycleSettingsDiffAction performActionsForCanvas:withUpdatedScene:settingsDiff:fromSettings:transitionContext:] + 231
    28  UIKit                               0x00000001072f2b25 -[_UICanvas scene:didUpdateWithDiff:transitionContext:completion:] + 392
    29  UIKit                               0x0000000106b5236a -[UIApplication workspace:didCreateScene:withTransitionContext:completion:] + 523
    30  UIKit                               0x000000010712d605 -[UIApplicationSceneClientAgent scene:didInitializeWithEvent:completion:] + 369
    31  FrontBoardServices                  0x000000011038dcc0 -[FBSSceneImpl _didCreateWithTransitionContext:completion:] + 338
    32  FrontBoardServices                  0x00000001103967b5 __56-[FBSWorkspace client:handleCreateScene:withCompletion:]_block_invoke_2 + 235
    33  libdispatch.dylib                   0x000000010a8a533d _dispatch_client_callout + 8
    34  libdispatch.dylib                   0x000000010a8aa9f3 _dispatch_block_invoke_direct + 592
    35  FrontBoardServices                  0x00000001103c2498 __FBSSERIALQUEUE_IS_CALLING_OUT_TO_A_BLOCK__ + 24
    36  FrontBoardServices                  0x00000001103c214e -[FBSSerialQueue _performNext] + 464
    37  FrontBoardServices                  0x00000001103c26bd -[FBSSerialQueue _performNextFromRunLoopSource] + 45
    38  CoreFoundation                      0x0000000104702101 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
    39  CoreFoundation                      0x00000001047a1f71 __CFRunLoopDoSource0 + 81
    40  CoreFoundation                      0x00000001046e6a19 __CFRunLoopDoSources0 + 185
    41  CoreFoundation                      0x00000001046e5fff __CFRunLoopRun + 1279
    42  CoreFoundation                      0x00000001046e5889 CFRunLoopRunSpecific + 409
    43  GraphicsServices                    0x000000010c9ae9c6 GSEventRunModal + 62
    44  UIKit                               0x0000000106b555d6 UIApplicationMain + 159
    45  [app name]                        0x00000001040b82bf main + 111
    46  libdyld.dylib                       0x000000010a921d81 start + 1
    )
    libc++abi.dylib: terminating with uncaught exception of type NSException

I solved this by doing the changes on the AppDelegate.M

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    self.window.backgroundColor = [UIColor whiteColor];

    vcMain *mainView = [[vcMain alloc] initWithNibName:@"vcMain" bundle:nil];
    /*
    UIViewController *mainView = [[UIViewController alloc] initWithNibName:@"vcMain" bundle:nil];
    */
    self.window.rootViewController = mainView;
    [self.window makeKeyAndVisible];
    return YES;
}

Answer 1

Try to remove btnClient outlet from Attribute Inspector and reassign outlet.