It looks like your edit form is a custom one. If you use a form component, Adalo will populate the values for each element in the form.
If you need to use custom forms and avoid the risk of not having a user assigned to the task, then have edit options depending on what you want to edit. “Edit the user”, “edit the project”, and “edit the description”. Each option takes the user to a specific form that only allows for editing that piece of information about the task and nothing else.