Graphics returning null in one place, but not another.

Question

If you want to compile and run my program.... you can get all the files here:
http://s94182144.onlinehome.us/randomstuff/files.zip
(The main classes you'll need to look at are DrawingCanvas, Trig and GraphEngine)
Here's a visual just in case:
http://s94182144.onlinehome.us/randomstuff/graph.jpg
When I click the "Graph It" button, I'm sending all those variables in the window to another class. Except the method I need to call is passed Graphics g... and thus, I needed to create a graphics variable. But it's returning null.
Here is my code in GraphEngine, which runs when a button is clicked. You'll notice for Graph It... I created a Graphics variable... this returns null. Except you'll also notice I do the same thing for "Clear"... but this works.
NOTE: I was getting mad so I named some methods not so nicely :p .... you may notice they are different in my files.
     public void actionPerformed(ActionEvent e)
          userDisplay=e.getActionCommand();
          if(userDisplay.equals("Draw"))
               String progChoice=grapher.graphType.getSelectedItem();
               System.out.println(progChoice);
               if(progChoice.equals("Lines"))
                    grapher.allButtons[0].enable();
                    draw();
               else if(progChoice.equals("Scatter Plot"))
                    lineOfBestFit();
          else if(userDisplay.equals("Graph It"))
               Trig.graphThisStuff();
               Graphics g = canvas.getGraphics();
               canvas.trig(g);
           else if(userDisplay.equals("Clear"))
                Graphics g = canvas.getGraphics();
               System.out.println(g);
                canvas.clear(g);
        else if(userDisplay.equals("Reset"))
            Trig.restartThis();   
          else if(userDisplay.equals("Credits"))
            showInstructionsFrame();
        else if(userDisplay.equals("Help"))
            showHelpFrame();   
        else if(userDisplay.equals("Create Sinusodial Graph"))
            showTrigFrame();   

Answer 1

1. Do you expect forum members to download and unzip your code? Good luck...
2. Inside almost every "big" problem is a small one trying to get out. Write a minimal program
demonstrating your problem (say <50 lines) and post that. The shorter code and the easier it
is to copy, paste and run, the more forum members will actually give it a go.
That being said, you problem is that you are using Component's getGraphics at all. Don't use it.
It doesn't work very well -- they rendering you do is temporary, if you iconify and restore your
window your changes will disappear (sometimes even moving another window past yours will do this!).
What should you do instead? Put all your rendering code in paintComponent (or subroutines
it calls). Changes in state of your app should trigger a call to repaint (or you will call it directly).
Repainting will eventually cause your paintComponent to be called.