Behind the scenes, when a VSTO addin or customization loads, the runtime uses reflection to search for all Ribbon controls and customizations and automatically adds them to the appropriate extension regions. This process makes it easy for developers as they don’t have to worry about hooking up the Ribbon Tab that they’ve developed to the Ribbon, or add a new menu item to the Office Menu. However, it can make the add-in’s startup time slower if your Office solution has many references and assemblies.

Ran into this issue when designing a Ribbon (Visual Designer) control that contained a ComboBox. After you add items to the ComboBox using the properties window, everything appears fine until you attempt to Save or Build. You will get the following error:

Code generation for property ‘Items’ failed. Error was: ‘Object reference not set to an instance of an object’