No SIM card installed keeps popping up

I have been using my wifi/cellular iPad mini without a SIM card for the last eight months without a problem. I 'upgraged' to os8 a couple of days ago and now I get a pop up telling me there's no SIM card installed every few minutes. It popped up twice whilst trying to type this message. I have looked everywhere in settings for a way to disable it, thinking that perhaps certain apps were prompting it. Will I have to put a sim card to avoid this? I was told when I bought it, that without a SIM card it would be just like a wifi only ipad. or will I have to reset it to factory settings and load everything again? Thanks in advance.