Same thing happened to me. Worker got to the field and started spinning around in circles. Went to tasks tab to delete the task and game crashed. I sent the crash report with an explanation on how it happened.

Also, when the worker completed the first mission at the beginning of the new game (got to the end of the field), the tractor stopped, the worker disappeared from inside the tractor, but then the tractor drove off on its own to the other corner of the field without a driver inside.

Same thing here really, Tasked a worker to plough a field, he takes the tractor and implement gets to the field and drives around in circles, when I cancelled the task the game crashed.

Crash report was sent.

Go to your Garage tab in the Menu and press the 'R' button on the AI controlled Tractor to reset the Worker. Then create a new Task instead of deleting the old Task.