Various fixes

[NicoPennec]

Problem

• Non-blocking errors may appear in the web console.
• In "My Checks" page, the "assignees" column can overflow if there are too many assignees.
• On the Task panel with 3D model, the background selector can be too large for the preview player.

Solution

• Fix silent errors.
• Fix overflow on assignees of My Checks page.
• Restyle background selector on 3D model player.