Exception in thread "main" java.lang.IllegalArgumentException: input == null! when using ImageIO.read(this.getClass().getResource()

Question

I got

Exception in thread "main" java.lang.IllegalArgumentException: input == null!
    at java.desktop/javax.imageio.ImageIO.read(ImageIO.java:1400)
    at studentsystem.frame.StudentSystemLoginFrame.<init>(StudentSystemLoginFrame.java:121)
    at studentsystem.run.TestLogin.main(TestLogin.java:7)

when I'm using Image img = ImageIO.read(this.getClass().getResource("/2.png")); The error appeared in StudentSystemLoginFrame. My project directory looks like this: