By default, all Dart variables and methods are non-nullable, meaning they can’t be or return null. So, to make a variable or method nullable, meaning to be or return null, you have to use the null-safety operator “?” on it datatype or before the variable or method.

You have to also use the symbol ! to check if a particular variable has a null value. If the value is null, Dart will throw an exception.

Example:

// Non-nullable ==> means it must have a value before using it (can't be null). 
String myName;
print(myName);  // this will be an error since is Non-nullable

// Nullable ==> means it can be null before using it
String? email;
print(email); // this print null, without causing any error

String school = null; // it will be error, you're assigning null value to a Variable that must not be null
String? church =null; // Correct no error, because it marked nullable to accept null values

String getName(){
  return null;  //error: the method is non-nulllable it must not return null
}

String? getTown(){
   return null; // correct: the method is marked nullable
}

List<String> colors1 =[null]; // error
List<String> colors2 =null;  //error
List<String?> newcolors =[null]; // correct
List<String>? colors =null;  //correct

Check also, How to solve null check operator used on a null value?

Use the “!” to check if a variable has a null value

void main(List<String> args) {
  // make the variable nullable but don't assign any value to it
  String? firstName;
  // use the "!" to check if the <firstName> has a value of null
  // if the value is not null, dart will not throw an exception
  //But if it has a value of null, dart will throw an exception like:
  // Null check operator used on a null value

  print(firstName!);
  /* result:
  Unhandled exception:
Null check operator used on a null value
*/

}