SUGGESTED FIX

--- awt_Window.cpp	2008-01-11 20:06:20.000000000 +0300
***************
*** 2143,2149 ****
  {
      TRY;
  
!     AwtWindow::wwindowPeerCls = cls;
      AwtWindow::getActiveWindowsMID =
          env->GetStaticMethodID(cls, "getActiveWindowHandles", "()[J");
      DASSERT(AwtWindow::getActiveWindowsMID != NULL);
--- 2143,2149 ----
  {
      TRY;
  
!     AwtWindow::wwindowPeerCls = (jclass)env->NewGlobalRef(cls);
      AwtWindow::getActiveWindowsMID =
          env->GetStaticMethodID(cls, "getActiveWindowHandles", "()[J");
      DASSERT(AwtWindow::getActiveWindowsMID != NULL);
                                     

EVALUATION

I tracked the crash up to the place where we get the array of active windows in AwtDialog:
void AwtDialog::ModalActivateNextWindow(HWND dialogHWnd,
                                        jobject dialogTarget, jobject dialogPeer){
    JNIEnv *env = (JNIEnv *)JNU_GetEnv(jvm, JNI_VERSION_1_2);
    jlongArray windows = (jlongArray)(env->CallStaticObjectMethod(AwtWindow::wwindowPeerCls, awtWindow::getActiveWindowsMID));

Seem that the AwtWindow::wwindowPeerCls member has Local instead of Global reference as its value. Searching the place where the assignment happens I found: 
Java_sun_awt_windows_WWindowPeer_initIDs(JNIEnv *env, jclass cls)
{
    TRY;
    AwtWindow::wwindowPeerCls = cls;
    AwtWindow::getActiveWindowsMID =...

Should wrap it into GlobalRef before using from the other thread.
Perhaps the change caused that code to be called on a different thread and caused this to happen went into early JDK7 build.
                                     

EVALUATION

_endModal() function exits with error code.