GeoJSON task creation/rebuild fails if number of tasks is divisible by 100

[wielandb]

To Reproduce

• Try to create a challenge using this GeoJSON tasks.json
• The page will say "100 tasks built", but this screen will never end
• Going into F12 reveals the API answered "KO: No json uploaded with request to add tasks from"
• After some time, go back to another MR page
• Go back to "Create & Manage"
• The challenge was still created and can be worked on, although the proccess never finished
• Try "rebuild tasks" with the exact same GeoJSON file
• The rebuilding will fail (at least in UI; sometimes the rebuild will still have happened in the backend, sometimes not)

Browser: Chrome, OS: Windows