Code causing NullPointerException

Question

On asking for a number, after entering, it is saying : Exception in thread "main" java.lang.NullPointerException at line ch[i].

Scanner scanner = new Scanner (System.in);
System.out.println("Enter a number");
int num = Integer.parseInt(scanner.nextLine());
//reverse number logic

int q = num / 10;
int r = num % 10;
num = q;
char ch[] = null;
int i = 1;

while(r > 0) {
  i++;
  ch[i] = (char) r ;
}