please initialize `self. install_psi_objdir` and `self. install_psi_srcdir` to `None` in a custom constructor (add this to the top of the easyblock):

```python
    def __init__(self, *args, **kwargs):
        """Initialize class variables custom to PSI."""
        super(EB_PSI, self).__init__(*args, **kwargs)

        self. install_psi_objdir = None
        self. install_psi_srcdir = None
```