I was getting the error: TypeError: Model 'project.task' does not exist in registry in my log file, then I removed that custom addon in which I inherit from project.task and it fixed my issue. HOWEVER I'd like to learn what I am doing wrong.
Log as well as python file are below.

LOG:

2019-01-06 19:46:36,497 3294 INFO bitnami_odoo odoo.modules.loading: loading 1 modules...
2019-01-06 19:46:36,509 3294 INFO bitnami_odoo odoo.modules.loading: 1 modules loaded in 0.01s, 0 queries
2019-01-06 19:46:36,510 3294 WARNING bitnami_odoo odoo.modules.graph: module library: not installable, skipped
2019-01-06 19:46:36,514 3294 WARNING bitnami_odoo odoo.modules.graph: module export_tasks: not installable, skipped
2019-01-06 19:46:36,532 3294 INFO bitnami_odoo odoo.modules.loading: loading 60 modules...
2019-01-06 19:46:36,534 3294 WARNING bitnami_odoo odoo.modules.loading: Transient module states were reset
2019-01-06 19:46:36,535 3294 ERROR bitnami_odoo odoo.modules.registry: Failed to load registry
Traceback (most recent call last):
  File "/opt/bitnami/apps/odoo/lib/odoo-11.0.post20180818-py3.6.egg/odoo/modules/registry.py", line 59, in __new__
    return cls.registries[db_name]
  File "/opt/bitnami/apps/odoo/lib/odoo-11.0.post20180818-py3.6.egg/odoo/tools/func.py", line 68, in wrapper
    return func(self, *args, **kwargs)
  File "/opt/bitnami/apps/odoo/lib/odoo-11.0.post20180818-py3.6.egg/odoo/tools/lru.py", line 44, in __getitem__
    a = self.d[obj].me
KeyError: 'bitnami_odoo'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/bitnami/apps/odoo/lib/odoo-11.0.post20180818-py3.6.egg/odoo/modules/registry.py", line 85, in new
    odoo.modules.load_modules(registry._db, force_demo, status, update_module)
  File "/opt/bitnami/apps/odoo/lib/odoo-11.0.post20180818-py3.6.egg/odoo/modules/loading.py", line 376, in load_modules
    force, status, report, loaded_modules, update_module, models_to_check)
  File "/opt/bitnami/apps/odoo/lib/odoo-11.0.post20180818-py3.6.egg/odoo/modules/loading.py", line 274, in load_marked_modules
    perform_checks=perform_checks, models_to_check=models_to_check
  File "/opt/bitnami/apps/odoo/lib/odoo-11.0.post20180818-py3.6.egg/odoo/modules/loading.py", line 146, in load_module_graph
    model_names = registry.load(cr, package)
  File "/opt/bitnami/apps/odoo/lib/odoo-11.0.post20180818-py3.6.egg/odoo/modules/registry.py", line 250, in load
    model = cls._build_model(self, cr)
  File "/opt/bitnami/apps/odoo/lib/odoo-11.0.post20180818-py3.6.egg/odoo/models.py", line 428, in _build_model
    raise TypeError("Model %r does not exist in registry." % name)
TypeError: Model 'project.task' does not exist in registry.

# -*- coding: utf-8 -*-

from odoo import models, fields, api

class ProjectTask(models.Model):
    _inherit = "project.task"

    date_deadline = fields.Date(related="sale_line_id.order_id.validity_date", string="Deadline", index=True, copy=False)

# -*- coding: utf-8 -*-
{
    'name': "Expiration to Deadline",
    'summary': """
        Set a sales order's expiration date to a task's deadline 
        """,
    'description': """
        This module assigns a sales order's expiration date to a project task's deadline. Of this way tasks show in the project's calendar.
    """,
    'author': "author",
    'website': "http://www.www.com",
    # Categories can be used to filter modules in modules listing
    # Check https://github.com/odoo/odoo/blob/master/odoo/addons/base/module/module_data.xml
    # for the full list
    'category': 'Uncategorized',
    'version': '0.1',
    # any module necessary for this one to work correctly
    'depends': ['base'],
    # always loaded
    'data': [
        # 'security/ir.model.access.csv',
        'views/views.xml',
        'views/templates.xml',
    ],
    # only loaded in demonstration mode
    'demo': [
        'demo/demo.xml',
    ],
}