Changing window-style to MDI or SDI may fail with IllegalComponentStateException

Javatest 4.2_b13

- Start the tool. Open or create workign directory.
- Open File/Preferences dialog
- change Window style from Tabbed (default) to MDI or SDI
- Press OK button

Preferences dialog is not closed. ExecTool (WD) in background is closed. Exception is thrown:
WARNING: missing resource: fakeToolDialog.desc for com.sun.javatest.exec.i18n
WARNING: missing resource: fakeToolDialog.name for com.sun.javatest.exec.i18n
Exception occurred during event dispatching:
java.awt.IllegalComponentStateException: contentPane cannot be set to null.
        at javax.swing.JRootPane.setContentPane(Unknown Source)
        at javax.swing.JInternalFrame.setContentPane(Unknown Source)
        at com.sun.javatest.tool.MDIDeskView.createDialog(MDIDeskView.java:273)
        at com.sun.javatest.tool.ToolDialog.initDialog(ToolDialog.java:497)
        at com.sun.javatest.tool.MDIDeskView.addTool(MDIDeskView.java:186)
        at com.sun.javatest.tool.MDIDeskView.<init>(MDIDeskView.java:81)
        at com.sun.javatest.tool.Desktop.setStyle(Desktop.java:228)
        at com.sun.javatest.tool.DesktopPrefsPane.save(DesktopPrefsPane.java:167
)
        at com.sun.javatest.tool.Preferences.setPreferences(Preferences.java:354
)

- press OK button again. This time Preferences dialog is closed.
- Window stype is changed, but ExecTool/WD is closed. User have to reopen it or restart the Harness.
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