Only dispose if display was actually created

Author: bmalinowsky

src/io/calimero/gui/Main.java

@@ -188,7 +188,8 @@ public static void main(final String[] args)
 		}
 		finally {
 			System.setErr(LogTab.oldSystemErr);
-			display.dispose();
+			if (display != null)
+				display.dispose();
 		}
 	}