[2.6, 2.7] crash in bedlevel-pattern for cartesio printer

[rburema]

Tested with Cura 5.1 and 5.8, plugin versions 2.6 and 'to approve' 2.7.

Found, (while smoke- & monkey-testing for UM Marketplace approval,) that if a Cartesio printer is selected (in 'MaukCC > Cartesio' under 'non UltiMaker printers') at least, the 'custom' bed level pattern option for 'circle-in-square' (no other options changed), would crash with the following stack-trace.

Traceback (most recent call last):
  File "C:\Users\r.burema\AppData\Roaming\cura\5.8\plugins\AutoTowersGenerator\AutoTowersGenerator\Controllers\BedLevelPatternContoller.py", line 59, in dialogAccepted
    self._generateCustomBedLevelPattern()
  File "C:\Users\r.burema\AppData\Roaming\cura\5.8\plugins\AutoTowersGenerator\AutoTowersGenerator\Controllers\BedLevelPatternContoller.py", line 96, in _generateCustomBedLevelPattern
    (print_area_width, print_area_depth) = self._dataModel.printArea
  File "C:\Users\r.burema\AppData\Roaming\cura\5.8\plugins\AutoTowersGenerator\AutoTowersGenerator\Models\ModelBase.py", line 88, in printArea
    minY = max([coord[1] for coord in flattenedList if coord[1] < 0])
ValueError: max() arg is an empty sequence

(As you can probably see from all the back-slashes in the filenames, I'm on Windows, should that matter 😁)