If you are not using .nib files, you can make sure that the code that creates your window is executed: window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
If you use .nib files (.xib), make sure that you properly delegate the window to the application delegate in the interface builder.
In my case, I was subclassed by the application delegate, and I accidentally deleted the line calling [super didFinishLaunchingWithOptions...]where my window was being created.
I hope this helps someone.
Greetings
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