Ignore polygons with length 0 when updating disallowed areas

Contributes to CURA-3148

cura/BuildVolume.py

@@ -285,6 +285,9 @@ class BuildVolume(SceneNode):
             color = Color(0.0, 0.0, 0.0, 0.15)
             for polygon in self._disallowed_areas:
                 points = polygon.getPoints()
+                if len(points) == 0:
+                    continue
+
                 first = Vector(self._clamp(points[0][0], min_w, max_w), disallowed_area_height, self._clamp(points[0][1], min_d, max_d))
                 previous_point = Vector(self._clamp(points[0][0], min_w, max_w), disallowed_area_height, self._clamp(points[0][1], min_d, max_d))
                 for point in points:
@@ -827,4 +830,4 @@ class BuildVolume(SceneNode):
     _tower_settings = ["prime_tower_enable", "prime_tower_size", "prime_tower_position_x", "prime_tower_position_y"]
     _ooze_shield_settings = ["ooze_shield_enabled", "ooze_shield_dist"]
     _distance_settings = ["infill_wipe_dist", "travel_avoid_distance", "support_offset", "support_enable", "travel_avoid_other_parts"]
-    _extruder_settings = ["support_enable", "support_interface_enable", "support_infill_extruder_nr", "support_extruder_nr_layer_0", "support_interface_extruder_nr", "brim_line_count", "adhesion_extruder_nr", "adhesion_type"] #Settings that can affect which extruders are used.
+    _extruder_settings = ["support_enable", "support_interface_enable", "support_infill_extruder_nr", "support_extruder_nr_layer_0", "support_interface_extruder_nr", "brim_line_count", "adhesion_extruder_nr", "adhesion_type"] #Settings that can affect which extruders are used.