why I have get this errorin dart?

this is my code and getting these error:

• The operator ‘<‘ can’t be unconditionally invoked because the receiver can be ‘null’.
• The argument type ‘int?’ can’t be assigned to the parameter type ‘num’.

code

 void main()
        {
          int romanToInt(String s) {
              s=s;
              Map<String,int> roman = {
                  'I':1,
                  'V':5,
                  'X':10,
                  'L':50,
                  'C':100,
                  'D':500,
                  'M':1000
                };
                int result=0;
                for(int i=0;i<s.length;i++){
                  if(i+1<s.length && roman[s[i]]<roman[s[i+1]])
                  {
                    result-= roman[s[i]];
                  }else{
                    result+= roman[s[i]];
                  }
                }
                print(result);
              return result;
            }
        }

>Solution :

Try this:

void main()
{
  int romanToInt(String s) {
    s=s;
    Map<String,int> roman = {
      'I':1,
      'V':5,
      'X':10,
      'L':50,
      'C':100,
      'D':500,
      'M':1000
    };
    int result=0;
    for(int i=0;i<s.length;i++){
      if(i+1<s.length && roman[s[i]]!<roman[s[i+1]]!)//<- add null check
      {
        result-= roman[s[i]]!; //<- add null check
      }else{
        result+= roman[s[i]]!;// <- add null check
      }
    }
    print(result);
    return result;
  }
}