Original PR description

Step to reproduce:
- Go to the Project module.
- Open all tasks
- Switch to Gantt view.
- Group tasks by Project.
- Observe the traceback error.

Cause:
- The error occurred after merging this [commit](https://github.com/odoo/odoo/pull/194673/commits/5c2e8c58290571a9d8e59e2602db6493b381945e), This change caused the `filter_domain_leaf()` function to return a `DomainBool` object, leading to the TypeError when attempting to use len() on it in the `_group_expand_project_ids` method.

Solution:
- To fix this, the `filter_domain_leaf()` output was wrapped with `list()`, ensuring that filtered_domain is always a list and allowing the safe use of `len()`.

task-4521684