package janelas;

public class desktop extends javax.swing.JFrame {
    private frameInterno register;

    public desktop() {
        initComponents();
    }

    private void initComponents() {

        painelDesktop = new javax.swing.JDesktopPane();
        btnAbrirFrame = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        javax.swing.GroupLayout painelDesktopLayout = new javax.swing.GroupLayout(painelDesktop);
        painelDesktop.setLayout(painelDesktopLayout);
        painelDesktopLayout.setHorizontalGroup(
            painelDesktopLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 0, Short.MAX_VALUE)
        );
        painelDesktopLayout.setVerticalGroup(
            painelDesktopLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 260, Short.MAX_VALUE)
        );

        btnAbrirFrame.setText("Abrir Frame");
        btnAbrirFrame.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnAbrirFrameActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(painelDesktop)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addContainerGap(301, Short.MAX_VALUE)
                .addComponent(btnAbrirFrame)
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addComponent(painelDesktop, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addComponent(btnAbrirFrame)
                .addGap(0, 11, Short.MAX_VALUE))
        );

        pack();
    }

    private void btnAbrirFrameActionPerformed(java.awt.event.ActionEvent evt) {
        if (register == null) {
            register = new frameInterno();
        }

        if (!register.isVisible()) {
            painelDesktop.add(register);
            register.setVisible(true);
        }
        register.toFront();
    }

    public static void main(String args[]) {
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                new desktop().setVisible(true);
            }
        });
    }

    private javax.swing.JButton btnAbrirFrame;
    private javax.swing.JDesktopPane painelDesktop;

}

package janelas;

public class frameInterno extends javax.swing.JInternalFrame {

    public frameInterno() {
        initComponents();
    }

    private void initComponents() {

        btnSair = new javax.swing.JButton();

        btnSair.setText("Sair");
        btnSair.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnSairActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addContainerGap(300, Short.MAX_VALUE)
                .addComponent(btnSair)
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addContainerGap(200, Short.MAX_VALUE)
                .addComponent(btnSair)
                .addContainerGap())
        );

        pack();
    }

    private void btnSairActionPerformed(java.awt.event.ActionEvent evt) {
        this.dispose();
    }

    private javax.swing.JButton btnSair;
}

Exception in thread "AWT-EventQueue-0"
java.lang.IllegalArgumentException: illegal component position  at
java.awt.Container.addImpl(Container.java:1100)     at
javax.swing.JLayeredPane.addImpl(JLayeredPane.java:231)     at
javax.swing.JDesktopPane.addImpl(JDesktopPane.java:484)     at
java.awt.Container.add(Container.java:417)  at
janelas.desktop.btnAbrirFrameActionPerformed(desktop.java:63)   at
janelas.desktop.access$000(desktop.java:3)  at
janelas.desktop$1.actionPerformed(desktop.java:31)  at
javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)    at
javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)    at
javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)     at
javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)  at
javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)  at
java.awt.Component.processMouseEvent(Component.java:6533)   at
javax.swing.JComponent.processMouseEvent(JComponent.java:3324)  at
java.awt.Component.processEvent(Component.java:6298)    at
java.awt.Container.processEvent(Container.java:2236)    at
java.awt.Component.dispatchEventImpl(Component.java:4889)   at
java.awt.Container.dispatchEventImpl(Container.java:2294)   at
java.awt.Component.dispatchEvent(Component.java:4711)   at
java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)  at
java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)   at
java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)   at
java.awt.Container.dispatchEventImpl(Container.java:2280)   at
java.awt.Window.dispatchEventImpl(Window.java:2746)     at
java.awt.Component.dispatchEvent(Component.java:4711)   at
java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)  at
java.awt.EventQueue.access$500(EventQueue.java:97)  at
java.awt.EventQueue$3.run(EventQueue.java:709)  at
java.awt.EventQueue$3.run(EventQueue.java:703)  at
java.security.AccessController.doPrivileged(Native Method)  at
java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)     at
java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)     at
java.awt.EventQueue$4.run(EventQueue.java:731)  at
java.awt.EventQueue$4.run(EventQueue.java:729)  at
java.security.AccessController.doPrivileged(Native Method)  at
java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)     at
java.awt.EventQueue.dispatchEvent(EventQueue.java:728)  at
java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)   at
java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)  at
java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)   at
java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)   at
java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)    at
java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

private void btnSairActionPerformed(java.awt.event.ActionEvent evt) {
    this.setVisible(false);
}

private void btnAbrirFrameActionPerformed(java.awt.event.ActionEvent evt) {
    if (register == null) {
        register = new frameInterno();
        painelDesktop.add(register);
    }

    if (!register.isVisible()) {
        register.setVisible(true);
    }
    register.toFront();
}