hr_timesheet_task : can't save new task : "Analytic Account" field is invalid

I am using an up-to-date OpenERP with OCB 7.0 branches for addons+server+web. Here is the scenario of the bug :

1) Create a new DB with demo data.
2) Install the module "project_timesheet"
3) Go to the menu Project > Project > Projects
4) Click on the project "The Jackson Group's Project" : you will arrive on the kanban vue of tasks
5) Create a new Task and, immediately after, create a new work item (don't do a "save" in between)
6 ) Click on Save => it works well

Now, Install the module hr_timesheet_task from lp:hr-timesheet

Run the same scenario :

3') Go to the menu Project > Project > Projects
4') Click on the project "The Jackson Group's Project" : you will arrive on the kanban vue of tasks
5') Create a new Task and, immediately after, create a new work item (don't do a "save" in between)
6') Click on Save
=> it doesn't work and the following message is displayed to the user in the top right corner : "The following fields are invalid" : "Analytic Account".