It's normal to post or mark the line that throws the exception.  As it is there's nothing obviously wrong with the code posted; though there's little need to include the fill(), rect() etc. calls in your constructor.  I instantiated a Scroll object and it works fine:

1. Scroll scroll;
2. void setup() {
3.   scroll = new Scroll();
4. }
5. void draw() {
6.   scroll.update();
7. }
8. 
   
9. class Scroll {
10.   float x ;
11.   float l ;
12.   Scroll()  {
13.      x = 0;
14.      l = x+width;
15.   }
16.  
17.   void update() {
18.     if(mouseX < x) {
19.        l = width - x - mouseX;
20.     }
21.      else {
22.         l = width + x + mouseX;
23.      }
24.      x = mouseX;
25.      print(l);
26.      background(0);
27.      
28.      fill(127);
29.      stroke(255);
30.      rect(0,10,width,20); 
31.      fill(255);
32.      stroke(0);
33.      rect(x,11,20,19);
34.   }
35. }

Could be an issue with the name of your tab (search the forum: this seems to be a recurring problem); or it could be a problem somewhere in the rest of your code...