Java SE (Java Platform, Standard Edition)

why does my program give me a NullPointerException when i press the CANCEL button? what can i do to make it close down the dialog when the cancel buttong is pressed?

thank you
Hristo

private String promptForSchoolName()
{
	String name = "";

	while(name.length() == 0)
	{
		name = JOptionPane.showInputDialog("Enter school name:");

		if(name.length() == 0)
		{
			JOptionPane.showMessageDialog(null, "Invalid input.");
		}
	}
	return name;
}