Please take a look the test (regression):
java/awt/Modal/LWModalTest/LWModalTest.java

InternalFrame is created and used in different threads without synchronization, this is the cause of NullPointerException.

java.lang.NullPointerException
 at LWModalTest.init(LWModalTest.java:89)
 at LWModalTest.main(LWModalTest.java:129)
 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
 at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
 at java.lang.reflect.Method.invoke(Method.java:601)
 at com.sun.javatest.regtest.MainWrapper$MainThread.run(MainWrapper.java:94)
 at java.lang.Thread.run(Thread.java:722)