Check to see if an SSD exists before trying to close it with the server

Fixes issue 3543
Fixes issue 3545

Change-Id: I71426cf26a029c2886c33a3c0698698031c0df95
Reviewed-on: http://gerrit.dmdirc.com/538
Automatic-Compile: Chris Smith <chris@dmdirc.com>
Reviewed-by: Chris Smith <chris@dmdirc.com>

src/com/dmdirc/addons/ui_swing/components/frames/ServerFrame.java

 import com.dmdirc.ui.interfaces.ServerWindow;
 import com.dmdirc.addons.ui_swing.components.SwingInputHandler;
 import com.dmdirc.addons.ui_swing.dialogs.serversetting.ServerSettingsDialog;
-import java.awt.Window;
 
 import java.awt.event.ActionEvent;
 import java.awt.event.ActionListener;
     /** {@inheritDoc} */
     @Override
     public void close() {
-        ServerSettingsDialog.getServerSettingsDialog(getContainer().getServer(),
+        if (ServerSettingsDialog.hasServerSettingsDialog()) {
+            ServerSettingsDialog.getServerSettingsDialog(getContainer().getServer(),
                 getController().getMainFrame()).dispose();
+        }
         super.close();
     }
 }

src/com/dmdirc/addons/ui_swing/dialogs/serversetting/ServerSettingsDialog.java

         return me;
     }
 
+    /**
+     * Checks if a Server settings dialog exists.
+     *
+     * @return true iif a dialog exists
+     */
+    public static boolean hasServerSettingsDialog() {
+        synchronized (ServerSettingsDialog.class) {
+            return me != null;
+        }
+    }
+
     /** Initialises the main UI components. */
     private void initComponents() {
         orderButtons(new JButton(), new JButton());