Fix NullPointerException in thread "Shutdown" - tray disposal - server mode

git-svn-id: http://svn.code.sf.net/p/davmail/code/trunk@2204 3d1905a2-6b24-0410-a738-b14d5a86fcbd

src/java/davmail/ui/tray/DavGatewayTray.java

@@ -291,6 +291,8 @@ public final class DavGatewayTray {
     }
 
     public static void dispose() {
-        davGatewayTray.dispose();
+        if (davGatewayTray != null) {
+            davGatewayTray.dispose();
+        }
     }
 }