There are two ways to create nullable variable.
1) Using var keyword,
Syntax : var variable_name;
2) Using datatype
Syntax : int? variable_name;

1. Nullable variable creation using var keyword.

void main() {
  var a;
  print(a);
  a = "Hello world";
  print(a);
}

Output:
null
Hello world

Explanation:
Here variable a is nullable type. Variable a can be null and any other datatype.

2. Nullable variable creation through type annotation.

void main() {
  int? a;
  print(a);
  a = 45;
  print(a);
}

Output:
null
45

Explanation:
Here variable a is nullable type. Variable a can be null and int type.