Cannot invoke "java.lang.Integer.intValue()" because the return value of "java.util.List.get(int)" is null when handling a form - spring

Question

I have a form where I have to display a checkbox for each step and an input to choose the position of the step.
So, I would like to add each checked step in the database, with the input also retrieved.
Here is my form:
<th:block th:each="etape: ${etapes}">
<input class="form-label mt-4" type="checkbox" name="etapes" th:field="*{etapes}" th:value="${etape.id}"/>
<label th:text="${etape.poste.name}"></label>
<input class="form-label mt-4" type="number" name="positionsPoste" th:field="*{positionsPoste}" th:placeholder="#{new.cheminementEtape.positionPoste.label}"
/>
<br>
</th:block>
and my service:
List<CheminementEtape> cheminementEtapes = new ArrayList<>();
for(int i=0; i<cheminementForm.getEtapes().size(); i++) {
for(int y=0; y<cheminementForm.getPositionsPoste().size(); y++){
CheminementEtape cheminementEtape = CheminementEtape.builder().disable(false).etape(cheminementForm.getEtapes().get(i)).cheminement(cheminement).positionPoste(cheminementForm.getPositionsPoste().get(y)).build();
cheminementEtapes.add(cheminementEtape);
}
}
this.cheminementEtapeService.add(cheminementEtapes);
return cheminement;
and my form:
#Data
public class CheminementForm {
#NotNull(message = "{cheminement.form.name.notEmpty}")
#Size(min=2, max=30)
private String name;
#NotNull(message = "{cheminementEtape.form.etape.notEmpty}")
private List<Etape> etapes;
#NotNull(message = "{cheminementEtape.form.positionPoste.notEmpty}")
private List<Integer> positionsPoste;
}
So I make two for loops, and for each checked step, I get the position input and I add it in the database with the retrieved values.
However, when I do this, I get an error:
Cannot invoke "java.lang.Integer.intValue()" because the return value of "java.util.List.get(int)" is null
at line 60
I don't understand... Any ideas ? thanks in advance