Nsinternalinconsistencyexception, Reason: Could Not Load Nib in Bundle

NSInternalInconsistencyException Could not load nib in bundle

It looks like you're trying to instantiate a nib called ThemePickerController.nib and it isn't present. Is the nib included as a project member?

NSInternalInconsistencyException', reason: 'Could not load NIB in bundle: 'NSBundle

This error can occur when you rename files outside of XCode.
To solve it you can just remove the files from your project (Right Click - Delete and "Remove Reference").

Then after you can re-import the files in your project and everything will be OK.

Custom cell is causing a 'NSInternalInconsistencyException', reason: 'Could not load NIB in bundle:

If xib name is ParentTableViewCell.xib Then you should do

itemsTable.register(UINib(nibName: "ParentTableViewCell", bundle: nil), forCellReuseIdentifier: "parentCell")

NSInternalInconsistencyException', reason: 'Could not load NIB in bundle:

see the difference of the following 3 ways to instantiate something from your Interface Builder work.

[NSStoryboard.mainStoryboard instantiateControllerWithIdentifier:@"youridentifier"];

NSStoryboard *launchScreen = [NSStoryboard storyboardWithName:@"LaunchScreen" bundle:NSBundle.mainBundle];
[launchScreen instantiateControllerWithIdentifier:@"yourIdentifier"];

[NSBundle.mainBundle loadNibNamed:@"SomeXibOrNib" owner:self topLevelObjects:nil];

In your example you try to load via the NSNib method but you obviously meant the NSStoryboard LaunchScreen.storyboard
With your code it is in fact looking for a Nib called "LaunchScreen.xib" or "LaunchScreen.nib" which the error tells you does not exist.

and your 2nd Question.
If your info.plist contains a proper entry for a MainStoryboard and this Storyboard is setup correctly, it will instantiate your Window with a ViewController as Root that was given.
Which means after the "Main" Window is loaded from that Storyboard you can access its rootViewController property asking for its ViewController in charge.

UI/NSViewControllers come usually with a corresponding NS/UIView for free, but because subclassed Controllers may be different a typecast is needed to get rid of the warning telling you that. So to make sure the variable rootController will hold a pointer to an object of ViewController that has a property view it is typecasted.
Which works because the given RootViewController is of type "ViewController" and very likely a subclass of NS/UIViewController.

Typecasts also express you know the inheritance works because you guarantee that a view property does exist at runtime.

PS:it is always a bit confusing when chosen classnames are very simplified without any further indication which one is meant. ViewController could have been named "ANViewController" which will help you in larger projects when you have a lot different ViewControllers.

Exception : 'NSInternalInconsistencyException', reason: 'Could not load NIB in bundle: 'NSBundle..... in Swift 4

While registert nib, nibName shoudbe Cell namee means CustomCellOneCollectionViewCell

collectionViewOne.register(UINib(nibName: "CustomCellOneCollectionViewCell", bundle: nil), forCellWithReuseIdentifier: "cell1")
collectionViewOne.delegate = self
collectionViewOne.datasource = self

and in cellForItemAt

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell1", for: indexPath) as! CustomCellOneCollectionViewCell
    return cell
}