Question: my code keeps giving me an error that says: Exception in thread "main" java.lang.IllegalStateException: Active words is null! is there any way to fix this?

import java.util.Set;

import java.util.TreeMap;

import java.util.ArrayList;

import java.util.Collections;

$/**$

$*$ Manages the details of EvilHangman. This class keeps

$*$ tracks of the possible words from a dictionary during

$*$ rounds of hangman, based on guesses so far.

$*$

$*/$

public class HangmanManager $\{$

$//$ instance variables $/$ fields

ArrayList activeWords;

private Set dictionary;

private int numGuessesAllowed;

private ArrayList guessesMade;

private HangmanDifficulty difficulty;

private String pattern;

private int numWrongGuesses;

$/**$

$*$ Create a new HangmanManager from the provided set of words and phrases.

$*$ pre: words $!=$ null, words.size$()>0$

$*$ @param words A set with the words for this instance of Hangman.

$*$ @param debugOn true if we should print out debugging to System.out.

$*/$

public HangmanManager$($Set words, boolean debugOn$)\{$

if $($words $==$ null $||$ words.size$()<=0)\{$

throw new IllegalArgumentException$("$Invalid set of words!"$)$;

$\}$

dictionary $=$ words;

$\}$

$/**$

$*$ Create a new HangmanManager from the provided set of words and phrases.

$*$ Debugging is off.

$*$ pre: words $!=$ null, words.size$()>0$

$*$ @param words A set with the words for this instance of Hangman.

$*/$

public HangmanManager$($Set words$)\{$

this$($words$,$ false$)$;

$\}$

$/**$

$*$ Get the number of words in this HangmanManager of the given length.

$*$ pre: none

$*$ @param length The given length to check.

$*$ @return the number of words in the original Dictionary

$*$ with the given length

$*/$

public int numWords$($int length$)\{$

int count $=0$;

for $($String word : dictionary$)\{$

if $($word$.$length$()==$ length$)\{$

count$++$;

$\}$

$\}$

return count;

$\}$

$/**$

$*$ Get for a new round of Hangman. Think of a round as a

$*$ complete game of Hangman.

$*$ @param wordLen the length of the word to pick this time.

$*$ numWords$($wordLen$)>0$

$*$ @param numGuesses the number of wrong guesses before the

$*$ player loses the round. numGuesses $>=1$

$*$ @param diff The difficulty for this round.

$*/$

public void prepForRound$($int wordLen, int numGuesses, HangmanDifficulty diff$)\{$

$//$ Resets variables to starting values to start new round

numGuessesAllowed $=$ numGuesses;

numWrongGuesses $=0$;

guessesMade $=$ new ArrayList$<>()$;

difficulty $=$ diff;

$//$ Fills active words list with words with inputted length

activeWords $=$ new ArrayList$<>()$;

fillWithGivenLength$($wordLen$)$;

pattern $=""$;

$//$ Fills pattern with $"-"$ to represent unknown letters

for $($int i $=0$; i $<$ wordLen; i$++)\{$

pattern $+=\text{'}-\text{'}$;

$\}$

$\}$

$/**$

$*$ The number of words still possible $($live$)$ based on the guesses so far.

$*$ Guesses will eliminate possible words.

$*$ @return the number of words that are still possibilities based on the

$*$ original dictionary and the guesses so far.

$*/$

public int numWordsCurrent$()\{$

$//$System$.$out.println$("$HERE$"+$ activeWords$)$;

if$($activeWords $==$ null$)\{$

throw new IllegalStateException$("$Active words is null!"$)$;

$\}$

return activeWords.size$()$;

$\}$

$/**$

$*$ Get the number of wrong guesses the user has left in

$*$ this round $($game$)$ of Hangman.

$*$ @return the number of wrong guesses the user has left

$*$ in this round $($game$)$ of Hangman.

$*/$

public int getGuessesLeft$()\{$

return numGuessesAllowed $-$ numWrongGuesses;

$\}$

$/**$

$*$ Return a String that contains the letters the user has guessed

$*$ so far during this round.

$*$ The characters in the String are in alphabetical order.

$*$ The String is in the form $[$let$1,$ let$2,$ let$3,...$ letN$].$

$*$ For example $[$a$,$ c$,$ e$,$ s$,$ t$,$ z$]$

$*$ @return a String that contains the letters the user

$*$ has guessed so far during this round.

$*/$

public String getGuessesMade$()\{$

StringBuilder s $=$ new StringBuilder$()$;

s$.$append$("[")$;

$//$ If user has made any guesses

if $($guessesMade$.$size$()>0)\{$

Collections.sort$($guessesMade$)$;

for $($int i $=0$; i $<$ guessesMade.size$()-1$; i$++)\{$

s$.$append$($guessesMade$.$get$($i$)+",")$;

$\}$

$//$ Append last element

s$.$append$($guessesMade$.$get$($guessesMade$.$size$()-1))$;

$\}$

$//$ Close off String

s$.$append$("]")$;

String string $=$ s$.$toString$()$;

return string;

$\}$

$/**$

$*$ Check the status of a character.

$*$ @param guess The characater to check.

$*$ @return true if guess has been used or guessed