Want to know the meaning of this operator ?. in dart?

I want to know the meaning of ?. operator in dart , if any one can explain it ? , thank you .
for example :
final length=names?.length;

>Solution :

That means that you are accessing a property that may be null. When you declare a variable but never set it, you do this:

String? testString; // the ? operator means that the variable can be null

This was added to prevent your program from crashing by null pointer exceptions.

I suggest you read Michael Horns comment which includes the documentation for null safety in Dart.