Exception error: Limit of jInternal Frames in NetBeans

There is a limit of jInternal frames on a jDesktopPane?
YES there is a limit of jInternal frames i.e. 20
Means you can add only 20 jInternal Frames on a jDesktop Pane. If you exceed the limit
then you see an event error or Class defination not find like that error:

Exception in thread "AWT-EventQueue-0" java.lang.NoClassDefFoundError: college/COLLEGE
at college.Login.Login(Login.java:257)
at college.Login.jButton1ActionPerformed(Login.java:221)
at college.Login.access$200(Login.java:25)
at college.Login$3.actionPerformed(Login.java:112)