Starting around 1AM PST on 8/20/2019, the UsedRange.Values property has started returning null.
Not on all Excel spreadsheets but on some, and we can't yet determine what factors cause it. Seems like a bug in the graph APIs, as this was working seamlessly for more than a year.
var getUsedRangeRequest = _sheetRequestBuilder.UsedRange().Request();
usedRange = getUsedRangeRequest.GetAsync().Result;
I expect usedRange.Values to be a non-null JToken and so we can access usedRange.Values.Children().ToList() .... etc.
Thanks for reporting this issue. This is recovered now. Could you please try again?