The stack trace precisely indicates where the error comes from:
Object[][] contents = getContents();
HashMap<String,Object> temp = new HashMap<>(contents.length);
for (int i = 0; i < contents.length; ++i) {
// key must be non-null String, value must be non-null
String key = (String) contents[i][0];
Object value = contents[i][1];
if (key == null || value == null) {
throw new NullPointerException(); // <-- HERE
}
temp.put(key, value);
}
So, you have your answer: although I haven't seen it mentioned in the documentation, ListResourceBundle
doesn't allow null values.