학교과제인데

문장입력하면 총 글자개수, 각 모음별 개수 보여주는 건데... 


자기가 원하는 만큼 계속해서 입력할 수 있고. 


다했는데.. 문제가 그만하고 싶을때 close버튼 누르면 자꾸 java. lang.NullPointerException이라는 오류가 발생해.. 



in extra credit lab4,  when i compile and run the program, it works. input message is showing as much as I want. I used infinite loop for this.   However, the problem is close button. when i press close button, it is saying "java.lang.NullPointerException". can you help which is the problem? 

BlueJ highlighted the part that   "for (i=0; i<inputString.length();i++)".

here is my code
--------------------------------------------------------------------------------------------------------------------------

import javax.swing.JOptionPane;

public class Vowel2
{
    public static void main(String[] args)
    {
        
        JOptionPane.showMessageDialog(null, "This program asks the user for a sentence,\n"
        +"searches the sentence for all vowels,\n"+"and displays the number of times each\n"
        +"vowel appears in the sentence.","",JOptionPane.INFORMATION_MESSAGE);
        
        String inputString;
        int aCount=0, iCount=0, uCount=0, eCount=0, oCount = 0;                                     
        int i;
        
        while(true)
        {
          inputString=JOptionPane.showInputDialog("Enter the sentence to search");

        for (i=0; i<inputString.length();i++)
        {
            if(inputString.charAt(i)=='A'||inputString.charAt(i)=='a')
            {
                aCount++;
            }
            if(inputString.charAt(i)=='I'||inputString.charAt(i)=='i')
            {
                iCount++;
            }
            if(inputString.charAt(i)=='U'||inputString.charAt(i)=='u')
            {
                uCount++;
            }
            if(inputString.charAt(i)=='E'||inputString.charAt(i)=='e')
            {
                eCount++;
            }
            if(inputString.charAt(i)=='O'||inputString.charAt(i)=='o')
            {
                oCount++;
            }        
        }
        
         String outputMessage= inputString+"\n\n"+"has "+inputString.length()+" characters."+"\n\nThere are\n"+aCount+" a's,\n"+eCount+" e's,\n"+iCount+" i's,\n"+oCount+
         " o's, and\n"+uCount+" u's.";
         JOptionPane.showMessageDialog(null,outputMessage,"", JOptionPane.INFORMATION_MESSAGE);
        }
    }
}
---------------------------------------------------------------------------------------------------------------------------




어디가 문제인가여??