To clear the Project Tree tab and all other tabs, use Close Project.
If a project is already open, the following warning will appear:
Selecting No will abort the sequence and return to the previous state of the program.
Selecting Yes will discard the current project and its contents (so it might be a good idea to save it first). The Project Tree tab then displays and empty project entitled 'new project...', whereas the other tabs are cleared completely.
The program is now ready for the user to create new datasets and analyses.