GridWorld Actor - Calling toString() causes a NPE

I am writing a custom Rock in GridWorld. However when, I run the following code:

    for(int i = 0;i<7;i++){
        Grid<Actor> g = getGrid();
        Location l = getLocation();
        int x = l.getCol();
        int y = l.getRow();
        switch(i){
        case 0:
            Location l1 = new Location(x-1,y-1);
            Actor a = g.get(l1);
            if((a.toString()).equals("Infectious Rock")){

            }else if((a.toString()).equals("Infectious Bug")){

            }else{
                a.removeSelfFromGrid();
            }

            break;

(This is repeated 7 more times with different variables and different coordinates)

Here is the NPE:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at infectiousRock.act(infectiousRock.java:18)

Does anyone know what is causing this?

You first have to check if the Actor you get from calling g.get(1l) exists or not. There is a simple fix to this, change your current if statement to:

if(a != null) {
    if((a.toString()).equals("Infectious Rock")){

    }else if((a.toString()).equals("Infectious Bug")){

    }else{
        a.removeSelfFromGrid();
    }
} else
    break;

Adding the extra !=null check should do the trick, and if not leave a comment and I'll do my best to update the answer.