New to Java

Im creating an array of turtles but I got this error
Exception in thread "main" java.lang.NullPointerException
at acm.graphics.GCanvas.add(GCanvas.java)
at acm.program.GraphicsProgram.add(GraphicsProgram.java)
at acm.program.GraphicsProgram.add(GraphicsProgram.java)
at TurtleRace_BK.run(TurtleRace_BK.java:18)
at acm.program.Program.runHook(Program.java)
at acm.program.Program.startRun(Program.java)
at acm.program.Program.start(Program.java)
at acm.program.Program.start(Program.java)
at acm.program.Program.main(Program.java)

what does that mean? I installed all my acm files correctly


here is part of my code

GTurtle[] turtle = new GTurtle[3];
		
add(turtle[0], 50, 50);
add(turtle[1], 100, 100);
add(turtle[2], 100, 200);
turtle[0].penDown();
turtle[1].penDown();
turtle[2].penDown();