NPE for missing icon

We have some kind of restucturing of our code base. Unfortunately there are some icons missing for some forms now. During runtime we only get this not very helpful message:

java.lang.NullPointerException
    at javax.swing.ImageIcon.<init>(ImageIcon.java:181)
    at x.x.x.$$$setupUI$$$(X.java)


Do you have a tip how to track which icon finally make the system crash? Unfortunately the icon location which made this error is the one which is null in the ImageIcon constructor.

Thanks
Thomas