Description of problem:
User is unable to open designer, if he previously opened model of active process instance.

Version-Release number of selected component (if applicable):
6.3.0.DR2

Steps to Reproduce:
1. Create process (Start -> User Task -> End)
2. Save process
3. Open project editor
4. Build Project
5. Go to process definitions
6. Start given process
7. Go to active process instances
8. In options of given process open 'Process Model'
9. Close shown model
10. Go back to project authoring
11. Try to open given process

Actual results:
In step 11. there is always opened project editor, user is not able to open designer for given process


Expected results:
In step 11. there user is able to open designer for given process