fix(mobile) : #MVS-134 fix mobile API NPE with timeslots sometimes being badly gotten because of multiple structures on one student.

[FlorentMr]

Describe your changes

Fix mobile API NPE with timeslots sometimes being badly gotten because of multiple structures on one student.

Checklist tests

cf ticket MA-1062

Issue ticket number and link

MVS-134 MA-1062

Checklist before requesting a review (magic string, indentation, comment/documentation...)

• [x] I have detailed the tests to do in my feature/fix in order to prevent consequents regressions (must specify in Checklist tests)
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation (API Doc etc...) - (must specify in Description for target version)
• [ ] If it is a consequent feature, I have added thorough tests.
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] Any dependent changes have been added to this project (must specify in Description)