### Steps to reproduce

1. Go to the projects list view

2. Create a new project list with a custom field filter (e.g. slack webhook URL) and save it (see screenshot)

3. Try opening the saved project list after it was saved


### What is the buggy behavior?

* the list cannot be viewed, an internal server error is shown instead


<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/158252/content">

### What is the expected behavior?

* the project list show open normally


### **Logs**

```ruby
app/models/queries/projects/filters/custom_field_context.rb:57:in `where_subselect_conditions': undefined method `user' for nil (NoMethodError)

allowed_project_ids = Project.allowed_to(context.user, :view_project_attributes)
^^^^^
from app/models/queries/filters/shared/custom_fields/base.rb:120:in `condition'
from app/models/queries/filters/shared/custom_fields/list_optional.rb:56:in `condition'
from app/models/queries/filters/shared/custom_fields/base.rb:105:in `where'
from app/models/queries/filters/base.rb:106:in `apply_to'
from app/models/queries/base_query.rb:191:in `block in apply_filters'
from app/models/queries/base_query.rb:190:in `each'
from app/models/queries/base_query.rb:190:in `inject'
from app/models/queries/base_query.rb:190:in `apply_filters'
from app/models/queries/base_query.rb:66:in `results'
```

### Screenshots and other files

New project list with custom field filter example:

<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/158244/content">

### Environment information

_Please check and fill out the following details to help us identify in what versions and distributions of OpenProject the error occurs_

**OpenProject installation type**

* [x] Hosted cloud edition
* [ ] Packaged installation
* What distribution?

* [ ] Docker All-in-one container
* [ ] Docker-compose installation
* [ ] Other (please specify)

**OpenProject version**

_v14.4_ Begin of the insertion

_First mentioned in support ticket 18566_End of the insertion