Hi, I am trying to delete a dashboard as an admin and it fails with the following stack in the backend. Any reason why\n```[SQL: SELECT dashboard_email_schedules.created_on AS dashboard_email_schedules_created_on, dashboard_email_schedules.changed_on AS dashboard_email_schedules_changed_on, dashboard_email_schedules.id AS dashboard_email_schedules_id, dashboard_email_schedules.active AS dashboard_email_schedules_active, dashboard_email_schedules.crontab AS dashboard_email_schedules_crontab, dashboard_email_schedules.recipients AS dashboard_email_schedules_recipients, dashboard_email_schedules.slack_channel AS dashboard_email_schedules_slack_channel, dashboard_email_schedules.deliver_as_group AS dashboard_email_schedules_deliver_as_group, dashboard_email_schedules.delivery_type AS dashboard_email_schedules_delivery_type, dashboard_email_schedules.dashboard_id AS dashboard_email_schedules_dashboard_id, dashboard_email_schedules.created_by_fk AS dashboard_email_schedules_created_by_fk, dashboard_email_schedules.changed_by_fk AS dashboard_email_schedules_changed_by_fk, dashboard_email_schedules.user_id AS dashboard_email_schedules_user_id\nFROM dashboard_email_schedules\nWHERE %(param_1)s = dashboard_email_schedules.dashboard_id]\n[parameters: {'param_1': 22}] \n(Background on this error at: <http://sqlalche.me/e/f405>)\nTraceback (most recent call last):\n  File "/home/ec2-user/superset/incubator-superset/venv/lib64/python3.6/site-packages/sqlalchemy/engine/base.py", line 1248, in _execute_context\n    cursor, statement, parameters, context\n  File "/home/ec2-user/superset/incubator-superset/venv/lib64/python3.6/site-packages/sqlalchemy/engine/default.py", line 590, in do_execute\n    cursor.execute(statement, parameters)\npsycopg2.errors.UndefinedColumn: column dashboard_email_schedules.slack_channel does not exist\nLINE 1: ...cipients AS dashboard_email_schedules_recipients, dashboard_...\n                                                             ^```\n```The above exception was the direct cause of the following exception:\n\nTraceback (most recent call last):\n  File "/home/ec2-user/superset/incubator-superset/venv/lib64/python3.6/site-packages/flask_appbuilder/models/sqla/interface.py", line 476, in delete\n    self.session.commit()\n  File "/home/ec2-user/superset/incubator-superset/venv/lib64/python3.6/site-packages/sqlalchemy/orm/scoping.py", line 162, in do\n    return getattr(self.registry(), name)(*args, **kwargs)\n  File "/home/ec2-user/superset/incubator-superset/venv/lib64/python3.6/site-packages/sqlalchemy/orm/session.py", line 1036, in commit\n    self.transaction.commit()\n  File "/home/ec2-user/superset/incubator-superset/venv/lib64/python3.6/site-packages/sqlalchemy/orm/session.py", line 503, in commit\n    self._prepare_impl()\n  File "/home/ec2-user/superset/incubator-superset/venv/lib64/python3.6/site-packages/sqlalchemy/orm/session.py", line 482, in _prepare_impl\n    self.session.flush()\n  File "/home/ec2-user/superset/incubator-superset/venv/lib64/python3.6/site-packages/sqlalchemy/orm/session.py", line 2496, in flush\n    self._flush(objects)\n  File "/home/ec2-user/superset/incubator-superset/venv/lib64/python3.6/site-packages/sqlalchemy/orm/session.py", line 2637, in _flush\n    transaction.rollback(_capture_exception=True)\n  File "/home/ec2-user/superset/incubator-superset/venv/lib64/python3.6/site-packages/sqlalchemy/util/langhelpers.py", line 69, in __exit__```