if (this.currentWorld != null)
        {
            //throw new RuntimeException("Already decorating!!"); // just comment this out
        }
        else
        {
            this.currentWorld = par1World;
            this.randomGenerator = par2Random;
            this.chunk_X = par3;
            this.chunk_Z = par4;
            this.decorate();
            this.currentWorld = null;
            this.randomGenerator = null;
        }
// or
//
        if(this.currentWorld == null)
        {
            this.currentWorld = par1World;
            this.randomGenerator = par2Random;
            this.chunk_X = par3;
            this.chunk_Z = par4;
            this.decorate();
            this.currentWorld = null;
            this.randomGenerator = null;
        }