Email User Invite not working

**Describe the bug**
When I invite a user via email, this feature is not working.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to User Management
2. Click on Send Invite
3. Fill in email
4. Click Send sign up request

**Expected behavior**
It shows "Invite sent" but no email has been received.

**Screenshots**

**Components/Services affected**

- [ ] API/Backend

**Additional context**
```
Log file of this:
` #Traceback (most recent call last):
 U File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 2077, in wsgi_app
 , response = self.full_dispatch_request()
 b File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 1525, in full_dispatch_request
 ' rv = self.handle_user_exception(e)
 e File "/usr/local/lib/python3.9/site-packages/flask_restful/__init__.py", line 271, in error_router
  return original_handler(e)
 e File "/usr/local/lib/python3.9/site-packages/flask_restful/__init__.py", line 271, in error_router
  return original_handler(e)
 g File "/usr/local/lib/python3.9/site-packages/flask_cors/extension.py", line 165, in wrapped_function
 E return cors_after_request(app.make_response(f(*args, **kwargs)))
 b File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 1523, in full_dispatch_request
 ! rv = self.dispatch_request()
 ] File "/usr/local/lib/python3.9/site-packages/flask/app.py", line 1509, in dispatch_request
 Q return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
 n File "/usr/local/lib/python3.9/site-packages/flask_jwt_extended/view_decorators.py", line 154, in decorator
 8 return current_app.ensure_sync(fn)(*args, **kwargs)
 A File "/app/code/utils/decorators.py", line 30, in func_wrapper
 ! return func(*args, **kwargs)
 = File "/app/code/api/user_api.py", line 602, in user_delete
  user.delete()
 6 File "/app/code/models/user.py", line 87, in delete
  db.session.commit()
 % File "", line 2, in commit
 ` File "/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/session.py", line 1435, in commit
 3 self._transaction.commit(_to_root=self.future)
 _ File "/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/session.py", line 829, in commit
  self._prepare_impl()
 f File "/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/session.py", line 808, in _prepare_impl
  self.session.flush()
 _ File "/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/session.py", line 3367, in flush
  self._flush(objects)
 ` File "/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/session.py", line 3507, in _flush
 2 transaction.rollback(_capture_exception=True)
 e File "/usr/local/lib/python3.9/site-packages/sqlalchemy/util/langhelpers.py", line 70, in __exit__
  compat.raise_(
 _ File "/usr/local/lib/python3.9/site-packages/sqlalchemy/util/compat.py", line 207, in raise_
  raise exception
 ` File "/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/session.py", line 3467, in _flush
  flush_context.execute()
 c File "/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/unitofwork.py", line 456, in execute
  rec.execute(self)
 c File "/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/unitofwork.py", line 630, in execute
 - util.preloaded.orm_persistence.save_obj(
 e File "/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/persistence.py", line 237, in save_obj
  _emit_update_statements(
 u File "/usr/local/lib/python3.9/site-packages/sqlalchemy/orm/persistence.py", line 1001, in _emit_update_statements
 c = connection._execute_20(
 e File "/usr/local/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1631, in _execute_20
 G return meth(self, args_10style, kwargs_10style, execution_options)
 p File "/usr/local/lib/python3.9/site-packages/sqlalchemy/sql/elements.py", line 325, in _execute_on_connection
 . return connection._execute_clauseelement(
 p File "/usr/local/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1498, in _execute_clauseelement
 ! ret = self._execute_context(
 j File "/usr/local/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1862, in _execute_context
 " self._handle_dbapi_exception(
 q File "/usr/local/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 2043, in _handle_dbapi_exception
  util.raise_(
 _ File "/usr/local/lib/python3.9/site-packages/sqlalchemy/util/compat.py", line 207, in raise_
  raise exception
 j File "/usr/local/lib/python3.9/site-packages/sqlalchemy/engine/base.py", line 1819, in _execute_context
  self.dialect.do_execute(
 f File "/usr/local/lib/python3.9/site-packages/sqlalchemy/engine/default.py", line 732, in do_execute
 * cursor.execute(statement, parameters)
 “sqlalchemy.exc.IntegrityError: (psycopg2.errors.NotNullViolation) null value in column "user_id" of relation "invite" violates not-null constraint
 ±DETAIL: Failing row contains (1, huydq2@xmail.com, 8e62d44ba153c5824aa73acbc9335768, null, 1, 1, t, 2022-08-23 07:43:27.472741+00, 2022-12-29 10:18:45.733978+00).


 _[SQL: UPDATE invite SET user_id=%(user_id)s, updated_at=now() WHERE invite.id = %(invite_id)s]
 0[parameters: {'user_id': None, 'invite_id': 1}]
 =(Background on this error at: https://sqlalche.me/e/14/gkpj)
```