OK.. I sent you the file link
I had been working along, everything was fine. I had set the density I liked and then, after setting up my render it started to fail. With the Null DB tag error. I have isolated the error down to a forest object. Which I have replaced a few times. It's a rather involved project so it takes some time to do.
I have been trying to rebuild my scene but not having much luck. Using the same sizes I was using before yields smaller plants for some reason. And I can no longer set the density to where I need it to be. But for the past couple of days it was all working well.
This time I rendered, then set THRESHOLD down from 50 to 20 and the null tag error started showing up. Consequently, raising the density X value and setting threshold to 50 again didn't resolve it. It fails every time now. But I don't get a warning about exceeding the number if objects.
What is threshold actually doing ? The manual only has one line of text for it.
I am using xrefs, importing from the library.