Visual Studio reporting errors (Value cannot be null) since last set of Xamarin updates applied

Since applying the latest Xamarin updates, Visual Studio is reporting the following error:

ERROR: Value cannot be null.
Parameter name: project

Unfortunately, with so much software auto-updating these days I cannot be 100% certain that it was the Xamarin update that caused it, but it coincided chronologically. Has anybody else seen this error, or have any clue as to what might be causing it / how to resolve it?

Many thanks,

John H.