[FIX] project: inability to save subtasks when project is added

Steps to reproduce:
- Access the 'project.task' module.
- Create a subtask.
- Attempt to save the subtask after adding a project to it.

Issue:
- Saving a subtask becomes impossible when a project is added, causing an
  obstruction in the process.

Solution:
- Rectify the functionality to enable successful saving of a subtask even after
  adding a project, ensuring a seamless process for subtask creation and
  association with projects.

task-3584963

Part-of: odoo/odoo#143282

addons/project/views/project_task_views.xml

@@ -337,6 +337,7 @@
                                    domain="[('active', '=', True), '|', ('company_id', '=', False), ('company_id', '=?', company_id)]"
                                    widget="project"
                             />
+                            <field name="display_in_project" invisible="True" force_save="1"/>
                             <field name="milestone_id"
                                 placeholder="e.g. Product Launch"
                                 context="{'default_project_id': project_id}"