allow null value to rgbToHex method

wise-webapp/src/main/java/com/wisemapping/exporter/FreemindExporter.java

@@ -357,6 +357,7 @@ public class FreemindExporter
 
     private String rgbToHex(String color) {
         String result = color;
+        if (color != null) {
             boolean isRGB = Pattern.matches("^rgb\\([0-9]{1,3}, [0-9]{1,3}, [0-9]{1,3}\\)$", color);
             if (isRGB) {
                 String[] rgb = color.substring(4, color.length() - 1).split(",");
@@ -365,6 +366,7 @@ public class FreemindExporter
                 Integer b = Integer.valueOf(rgb[2].trim());
                 result = String.format("#%02x%02x%02x", r, g, b);
             }
+        }
         return result;
     }