The argument type 'int?' can't be assigned to the parameter type 'num'.
when using darts with 'null safety', this code gets an error but when I use the version of darts without null safety the test runs successfully (without error). My question is how do I change this code so that it doesn't error on darts with null safety?
If you take a look at the documentation of the [] operator on Map:
V? operator [](
Object? key
)
V? here means that the type of the value V will always be nullable. The reason for this is because of the following detail for this operator:
The value for the given
key, ornullifkeyis not in the map.
https://api.dart.dev/stable/2.18.1/dart-core/Map/operator_get.html
So the reason for your error is because your code is not prepared for the case of [] returns null in case the key are not in your Map.
There are multiple solutions for this problem which depends on how sure you are about the specific piece of code and what behavior you want in case of failure.
If you are e.g. completely sure that curr will always point to a valid key in the Map you can do: scarScore[curr]! (notice the !). This will add a runtime null-check and crash your application in case the value from the [] operator ends up being null.
Alternative, if you expect some keys to not be in the Map, you can provide default values like prev + (scarScore[curr] ?? 0) where null will then be assumed to be translated to the value 0.
For a detailed explanation for why the [] operator are nullable, you can read: https://dart.dev/null-safety/understanding-null-safety#the-map-index-operator-is-nullable
