Some where in your code (not in the file you gave) you are accessing access_token somewhat like this: some-object.access_token

But, some-object might be null as well. So to avoid the error you could use optional chaining like this: some-object?.access_token

The question mark "?" is for the optional chaining. it will first check if "some-object" is present or not and then it will try to access "access_token"