Impacted versions:
9.0 Community & 9.0 Enterprise

Steps to reproduce:
- Open up a V9 runbot
- Go to projects and click on the issues item, for example for the project 'Research & Development':
  ![image](https://cloud.githubusercontent.com/assets/6352350/13873686/65f2c31c-ecf3-11e5-917a-38df2832bd00.png)
- The project tasks view is opened in place of the project issues. (notice that this only happens every once in a while, about once in five attempts) Example:
  ![image](https://cloud.githubusercontent.com/assets/6352350/13873789/14798b64-ecf4-11e5-9b77-f2ad5d3a8adc.png)

Current behavior:
Every once in a while (about one out of five attempts) after clicking on the issues button it will open up the project tasks, not the issues.

Expected behavior:
When you click on the issues the view with all issues opens, not the view for all tasks.

addons/project_issue/project_issue.py
"""
        @param cr: the current row, from the database cursor,
        @param uid: the current user’s ID for security checks,
        @param ids: List of Openday’s IDs
        @return: difference between current date and log date
        @param context: A standard dictionary for contextual values
        """
        Calendar = self.pool['resource.calendar']
        res = dict((res_id, {}) for res_id in ids)
        for issue in self.browse(cr, uid, ids, context=context):
            values = {
                'day_open': 0.0, 'day_close': 0.0,
                'working_hours_open': 0.0, 'working_hours_close': 0.0,
                'days_since_creation': 0.0, 'inactivity_days': 0.0,
            }
            # if 
            calendar_id = None
            if issue.project_id and issue.project_id.resource_calendar_id: