Pretty self-explanatory:
Go into editor, create new osu!mania map, it'll cause an error, but you'll be able to continue. However, try to place any object - and osu! will crash.
The same applies when you try to do something on existing map - place one note - and error pop-ups without crash, but place next note and osu! crashes.
Error:
System.NullReferenceException: Object reference not set to an instance of an object.
Go into editor, create new osu!mania map, it'll cause an error, but you'll be able to continue. However, try to place any object - and osu! will crash.
The same applies when you try to do something on existing map - place one note - and error pop-ups without crash, but place next note and osu! crashes.
Error:
System.NullReferenceException: Object reference not set to an instance of an object.