Libgdx Tiled | Using setTile results in a NullPointerException

Question

So, I'm attempting to change the texture of a block when the player comes in contact with it by using cell.setTile(tileSet.getTile(926). I've already looked at similar questions and I guess the layer in Tiled is coming up as null, but I have no clue how to fix it.

I'm sorry it's a long post.

Here's the World Creator

public class B2DWorldCreator {

public B2DWorldCreator(PlayScreen screen) {
    World world = screen.getWorld();
    TiledMap map = screen.getMap();

    BodyDef bdef = new BodyDef();
    FixtureDef fdef = new FixtureDef();
    PolygonShape shape = new PolygonShape();
    Body body;

    //create static bodies for every wall in the game
    for (RectangleMapObject object : map.getLayers().get(2).getObjects().getByType(RectangleMapObject.class)){
        new Wall(screen, object);


    }


}
}

Here's the Tile class

public class Tile {
protected World world;
protected TiledMap map;
protected Rectangle bounds;
protected Body body;
protected PlayScreen screen;
protected MapObject object;
protected Fixture fixture;

public Tile(PlayScreen screen, MapObject object) {
    this.screen = screen;
    this.map = screen.getMap();
    this.world = screen.getWorld();
    this.object = object;
    bounds = ((RectangleMapObject) object).getRectangle();

    BodyDef bdef = new BodyDef();
    FixtureDef fdef = new FixtureDef();
    PolygonShape shape = new PolygonShape();


    bdef.type = BodyDef.BodyType.StaticBody;
    bdef.position.set(bounds.getX() + bounds.getWidth() / 2, bounds.getY() + bounds.getHeight() / 2);

    shape.setAsBox(bounds.getWidth() / 2, bounds.getHeight() / 2);
    fdef.shape = shape;

    body = world.createBody(bdef);
    fixture = body.createFixture(fdef);
}

public void setCategoryFilter(short filterBit) {
    Filter filter = new Filter();
    filter.categoryBits = filterBit;
    fixture.setFilterData(filter);

}
}

Here's the wall class where I call the problematic line at 31.

public class Wall extends Tile {
private TiledMapTileSet tileSet;
private TiledMapTileLayer layer;
private TiledMapTileLayer.Cell cell;

public Wall(PlayScreen screen, MapObject object){
    super(screen, object);
    fixture.setUserData(this);
    setCategoryFilter(AdventureCompany.WALL_BIT);
    tileSet = map.getTileSets().getTileSet("PlaceHoldernoTree.tsx");

}

public void onContact(){
    cell = new TiledMapTileLayer.Cell();
    if(object.getProperties().containsKey("Door")){
        layer = (TiledMapTileLayer) map.getLayers().get(1);
        cell = layer.getCell((int) body.getPosition().x / 16, ((int) body.getPosition().y / 16));
        System.out.println(cell.getTile());
        cell.setTile(tileSet.getTile(926));
    }

}
}

My Contact Listener

public class WorldContactListener implements ContactListener {
private AssetManager manager;


public WorldContactListener(AssetManager manager) {
    this.manager = manager;
}

@Override
public void beginContact(Contact contact) {
    //these are what came in contact
    Fixture fixA = contact.getFixtureA();
    Fixture fixB = contact.getFixtureB();

    int cDef = fixA.getFilterData().categoryBits | fixB.getFilterData().categoryBits; // this will be used to activate specific collision results
    Gdx.app.log("Contact", "ya boi");
    switch(cDef) {
        case AdventureCompany.EXPLORER_BIT | AdventureCompany.WALL_BIT:
            if(fixA.getFilterData().categoryBits == AdventureCompany.WALL_BIT)
                ((Wall) fixA.getUserData()).onContact();
            else
                ((Wall) fixB.getUserData()).onContact();


            break;
    }


}

And finally, the Tmx file header

    <?xml version="1.0" encoding="UTF-8"?>
<map version="1.0" tiledversion="1.1.5" orientation="orthogonal" renderorder="right-down" width="64" height="64" tilewidth="16" tileheight="16" infinite="0" nextobjectid="135">
 <tileset firstgid="1" source="test6.tsx"/>
 <tileset firstgid="925" source="PlaceHoldernoTree.tsx"/>
 <layer name="Background" width="64" height="64">
  <data encoding="base64">

Answer 1

After a lot of headache, I figured out that my problem was that in my Wall class when I did tileSet = map.getTileSets().getTileSet("PlaceHoldernoTree.tsx"); the file type was unnecessary and only needed the file name "PlaceHoldernoTree".