Automatically create menu when it does not exists

Background

When using a new channel or a new theme and one of the menus have not been used before, an error is raised (see comment).

Requirements

• Create a menu when it does not exist (instead of the exception that is thrown).

Test

• Create a new channel and add a website connector, choosing a theme with menus.
• The menu should be editable.
• The edits should be saved in the right channel.