NPE in DataDictionary::isTrailerField

In class DataDictionary: if expression "messageFields.get(TRAILER_ID)" returns null - NPE will be thrown.

I suggest to write this method just like isHeaderField method.

public boolean isTrailerField(int field) {
if (messageFields.get(TRAILER_ID) == null)

return messageFields.get(TRAILER_ID).contains(field);
}