Solving Xaml Internal Error error WMC9999

There I was, just devving along, when I hit F5 and this error pops up:

C:\Program Files (x86)\MSBuild\Microsoft\WindowsXaml\v14.0\8.2\Microsoft.Windows.UI.Xaml.Common.targets(264,5): 
Xaml Internal Error error WMC9999: Object reference not set to an instance of an object.

What does that even mean?
Google shows me a variety of things that may cause this.
But none of them helped me.

When reviewing my changes since the last check-in, I found this:

Notice anything odd?
Yes, that’s a binding on a click. Stupid copy-paste!
Anyway, changing that Click to Command dit the trick!

Thank you VS for not giving me a proper error…