[FIX] web_editor: impossible to add signature at some times

Before this commit:

When we try to add signature from powerbox then sometimes it does not show
the signature.

After this commit:

Now when we try to add signature from powerbox then it will show the signature.

Task-2939080

closes odoo/odoo#103013

Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>