Flutter/Dart: Why does this statement effect null-nafety?

Question

I realized this is just fine:

List<int> list = [];
int? b;
void addBarIfNull() {
  if (b != null) {
    list.add(b);  // no problem
  }
}

But adding this one statement b = null after adding b to the list, lets the linter complain with: The argument type 'int?' can't be assigned to the parameter type 'int':

List<int> list = [];
int? b;
void addBarIfNull() {
  if (b != null) {
    list.add(b);  // problem: The argument type 'int?' can't be assigned to the parameter type 'int'
    b = null;
  }
}

Can someone explain what is going on here? Why is b considered as int? in list.add(b) if we clearly check before that it is not null? Why do both code snippets differ in handling this?

Answer 1

    It is null safey issue when you declare 
{
int? b; 

}
you allow b to be null or have value but when you try to add
{
 list.add(b);  
}
b to the list you don't allow the list to contain null value that why the error message said you can't add int? to int you need to allow the list to contain null value if that whta you needed.

{
List<int?> list =[];
}