exampleSet.getExampleFromId returns null

I want to retrieve an example object by the "exampleSet.getExampleFromId" method, but got an NullPointerException error, anybody can tell me how to use this method?

Sample source likes this (Simplified Version)
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Set<Double> exampleIds = new HashSet<Double>(exampleSet.size());

for (Example example: exampleSet) {
exampleIds.add(example.getId());
}

for (Double exampleId: exampleIds) {
Example example = exampleSet.getExampleFromId(exampleId);
example.getWeight();
}
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