unchecked_use_of_nullable_value

A nullable expression can't be used as a condition.

A nullable expression can't be used as an iterator in a for-in loop.

A nullable expression can't be used in a spread.

A nullable expression can't be used in a yield-each statement.

The function can't be unconditionally invoked because it can be 'null'.

The method '{0}' can't be unconditionally invoked because the receiver can be 'null'.

The operator '{0}' can't be unconditionally invoked because the receiver can be 'null'.

The property '{0}' can't be unconditionally accessed because the receiver can be 'null'.

The analyzer produces this diagnostic when an expression whose type is potentially non-nullable is dereferenced without first verifying that the value isn't null.

The following code produces this diagnostic because s can be null at the point where it's referenced:

void f(String? s) {
  if (s.length > 3) {
    // ...
  }
}

If the value really can be null, then add a test to ensure that members are only accessed when the value isn't null:

void f(String? s) {
  if (s != null && s.length > 3) {
    // ...
  }
}

If the expression is a variable and the value should never be null, then change the type of the variable to be non-nullable:

void f(String s) {
  if (s.length > 3) {
    // ...
  }
}

If you believe that the value of the expression should never be null, but you can't change the type of the variable, and you're willing to risk having an exception thrown at runtime if you're wrong, then you can assert that the value isn't null:

void f(String? s) {
  if (s!.length > 3) {
    // ...
  }
}