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This happens because you saved in a cave or were very recently in a cave. Save files are located at [your user]\AppData\LocalLow\Ravensong Games\Axu\Save. It can be fixed. 

Open the save file in notepad and search for "PGLm"; it should be near the very bottom of the save file. Look along this section and you will see a repeating list of text that looks like so: {"P":[104,95,0],"LTS":83376,"Ch":[],"N":""}. The specific numbers within will be different but the format will be like this. You will find the first string after the word "Local". Scan through the list of strings that look like the above. If you see any with a positive third number in the square brackets, add a minus sign in front of it. 

For example, if you see {"P":[105,95,1],"LTS":84579,"Ch":[],"N":""}, change the 1 to a -1 like so: {"P":[105,95,-1],"LTS":84579,"Ch":[],"N":""}. Check the entire list of these strings and fix any you see with that positive third number, and the save file should work again.

I peeled apart a save file, and this seems to happen if you save when you are below ground or if you were very recently below ground. The recommendation of wandering around in the (ground floor) wilderness for a large chunk of turns is a good one as it clears out any improperly saved below-ground locations. From what I can guess, the error "NullReferenceException: Object reference not set to an instance of an object" is because the game is trying to reference a floor of a dungeon/cave that doesn't exist.