Unhandled exception EmptyStackException in QueueUsingTwoStacks.java

[aitorfi]

Describe the bug
The program should print "null" to show that the inStack is empty but instead the program crashes when trying to access an element of the empty stack.

Screenshots

[aitorfi]

I'd like to work on this issue.

[siriak]

This is expected behavior because Peek should return the element at the top of the stack, but there isn't any. This is consistent with other implementations like https://docs.oracle.com/javase/7/docs/api/java/util/Stack.html#peek()