What does it mean when the return value is null?

The documentation does not mention that this can happen. And what it means.

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#### Document Details

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* ID: d8316ba8-bc82-0353-e185-a50224e6ff5b
* Version Independent ID: f533a50a-55fb-e2f2-f379-b9afe686ee8c
* Content: [IItemContainerGenerator.GenerateNext Method (System.Windows.Controls.Primitives)](https://docs.microsoft.com/en-us/dotnet/api/system.windows.controls.primitives.iitemcontainergenerator.generatenext?f1url=https%3A%2F%2Fmsdn.microsoft.com%2Fquery%2Fdev15.query%3FappId%3DDev15IDEF1%26l%3DEN-US%26k%3Dk(System.Windows.Controls.Primitives.IItemContainerGenerator.GenerateNext);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.7.2);k(DevLang-csharp)%26rd%3Dtrue&view=netframework-4.7.2)
* Content Source: [xml/System.Windows.Controls.Primitives/IItemContainerGenerator.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Windows.Controls.Primitives/IItemContainerGenerator.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**