Clearing theme colours breaks SCSS compilation

When editing theme colours and clearing the input fields, the defaults are nto applied back. This causes the re-compilation to break, anbd AlekSIS to lose all style:

ERROR 2022-01-11 10:27:14,707 processor: Error: error in C function get-colour: 
       Traceback (most recent call last):
         File "/home/nik/AlekSIS/AlekSIS/apps/official/AlekSIS-App-Resint/.venv/lib/python3.9/site-packages/sass.py", line 167, in __call__
           return self.callable_(*args, **kwargs)
         File "/home/nik/AlekSIS/AlekSIS/apps/official/AlekSIS-App-Resint/.venv/lib/python3.9/site-packages/aleksis/core/util/sass_helpers.py", line 12, in get_colour
           rgb = web2hex(html_colour, force_long=True)[1:]
         File "/home/nik/AlekSIS/AlekSIS/apps/official/AlekSIS-App-Resint/.venv/lib/python3.9/site-packages/colour.py", line 675, in web2hex
           raise ValueError("%r is not a recognized color." % web)
       ValueError: '' is not a recognized color.
        on line 37:30 of .venv/lib/python3.9/site-packages/aleksis/core/static/public/theme.scss, in function `get-colour`
        from line 37:30 of .venv/lib/python3.9/site-packages/aleksis/core/static/public/theme.scss
        from line 8:9 of .venv/lib/python3.9/site-packages/aleksis/core/static/public/materialize-custom.scss
        from line 1:9 of .venv/lib/python3.9/site-packages/aleksis/core/static/public/style.scss
>> $primary-color: adjust-color(get-colour(get-preference(theme, primary)), $al
   -----------------------------^