Why does a non-empty List throw a Null Pointer Exception?

I have an List array 'details'.

List<HistoryDetails> details

where HistoryDetails is an object containing strings.

When I check the size it gives me a positive value.

details.size()

But when I try to access an element it throws a null pointer exception.

details.get(0).getFirstElement();


Attempt to invoke virtual method 'java.lang.String com.HistoryDetails.getFirstElement()' on a null object reference

I am invoking the element at the very next line to where I check the size. Hence, nothing should be reset. What could be wrong?