I have seen this intermittently as well, and talked about it a little bit while reporting another issue. (Apparently my account is too new to link to it? I'm getting flagged as spam, so the thread name is: "Can not place airport: Object reference not set to an instance of an object".)
The one time I thought I had caught it, I increased a bus line's number of vehicles by 2 and got 2 Null Reference Exception dialogs which seemed to coincide with each new bus spawning in. I was not able to confirm this for sure, and cannot reproduce the issue right now by adding new vehicles.
No mods, and the city was created after the 1.17.1 update.
DLCs:
* Green Cities
* Industries
* Mass Transit
* Parklife
* After Dark
* Snowfall
* Sunset Harbor
* Plazas and Promenades
* Cont Creator Pack: Bridge and Piers
Update: This happened again. This time, it was when I switched a line from regular busses to Articulated Biofuel busses. I got one error dialog with the Null Reference Exception every time a bus spawned in. I've attached the save and output_log.txt. I saved the game after a few error messages popped up, but pretty much immediately after changing the busses and getting the error message, and I set the game to close after saving.
