[stable4.7] fix: delay initializing the resize observer until FC is ready

[backportbot[bot]]

Backport of #5995

Warning, This backport's changes differ from the original and might be incomplete ⚠️

Todo

• [ ] Review and resolve any conflicts

• [ ] Amend HEAD commit to remove the line stating to skip CI

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[st3iny]

Is not required as the bug is not present on stable4.7.