[FIX] project: sub-tasks

- Activate sub-tasks in projects
- Enter a project's tasks, click on the stat button 'Sub-tasks'
- Reload the page (F5)

A traceback occurs since `project_id` is undefined.

The fields `project_id` and `partner_id` are only defined in the context
of the stat button.

opw-1844334

addons/project/models/project.py

@@ -877,6 +877,20 @@ class Task(models.Model):
             'type': 'ir.actions.act_window'
         }
 
+    def action_subtask(self):
+        action = self.env.ref('project.project_task_action_sub_task').read()[0]
+        ctx = self.env.context.copy()
+        ctx.update({
+            'default_parent_id' : self.id,
+            'default_project_id' : self.env.context.get('project_id', self.subtask_project_id.id),
+            'default_name' : self.env.context.get('name', self.name) + ':',
+            'default_partner_id' : self.env.context.get('partner_id', self.partner_id.id),
+            'search_default_project_id': self.env.context.get('project_id', self.subtask_project_id.id),
+        })
+        action['context'] = ctx
+        action['domain'] = [('id', 'child_of', self.id), ('id', '!=', self.id)]
+        return action
+
 
 class AccountAnalyticAccount(models.Model):
     _inherit = 'account.analytic.account'

addons/project/views/project_views.xml

@@ -84,16 +84,6 @@
             <field name="name">Sub-tasks</field>
             <field name="res_model">project.task</field>
             <field name="view_mode">kanban,tree,form,calendar,pivot,graph</field>
-            <field name="domain">[('id', 'child_of', active_id), ('id', '!=', active_id)]</field>
-            <field name="context">
-            {
-                'default_parent_id' : active_id,
-                'default_project_id' : project_id,
-                'default_name' : name + ':',
-                'default_partner_id' : partner_id,
-                'search_default_project_id': [project_id],
-            }
-            </field>
             <field name="search_view_id" ref="project.view_task_search_form"/>
         </record>
 
@@ -403,7 +393,7 @@
                     <sheet string="Task">
                     <div class="oe_button_box" name="button_box">
                         <button class="oe_stat_button" icon="fa-tasks" type="object" name="action_open_parent_task" string="Parent Task" attrs="{'invisible' : [('parent_id', '=', False)]}" groups="project.group_subtask_project"/>
-                        <button name="%(project_task_action_sub_task)d" type="action" class="oe_stat_button" icon="fa-tasks"
+                        <button name="action_subtask" type="object" class="oe_stat_button" icon="fa-tasks"
                             attrs="{'invisible' : [('parent_id', '!=', False)]}"
                             context="{'project_id': subtask_project_id, 'name': name, 'partner_id': partner_id}" groups="project.group_subtask_project">
                             <field string="Sub-tasks" name="subtask_count" widget="statinfo"/>