Time Series Forecasting
I have not been able to reproduce the blank model fit results.  But, I know that the forecast function will only forecast up to known xreg inputs. For instance, your test period starts at 1/2019 and ends 12/2019 (12 periods) however you're attempting to forecast  72 periods.  Try setting the h parameter in the forecast function equal to 12 and see if you get the same result.

To get around this problem in the past, I have forecasted the xreg series into the future (in your case an additional 60 periods for each of the four xreg variables).  You'll then need to add these 60 periods to the end of the test set for each respective variable.