filter satins for auto route satin (#2389)

lib/elements/satin_column.py

@@ -830,6 +830,8 @@ class SatinColumn(EmbroideryElement):
     def center_line(self):
         # similar technique to do_center_walk()
         center_walk = [p[0] for p in self.plot_points_on_rails(self.zigzag_spacing, (0, 0), (-0.5, -0.5))]
+        if len(center_walk) < 2:
+            center_walk = [center_walk[0], center_walk[0]]
         return shgeo.LineString(center_walk)
 
     def offset_points(self, pos1, pos2, offset_px, offset_proportional):

lib/extensions/auto_satin.py

@@ -63,7 +63,12 @@ class AutoSatin(CommandsExtension):
         starting_point = self.get_starting_point()
         ending_point = self.get_ending_point()
 
-        # Ignore fills
-        elements = [element for element in self.elements if isinstance(element, SatinColumn) or isinstance(element, Stroke)]
+        # Ignore fills and zero length satins
+        elements = [element for element in self.elements if (isinstance(element, SatinColumn) and element.center_line.length != 0) or
+                    isinstance(element, Stroke)]
+
+        # at this point we possibly removed all the elements, in this case stop here
+        if not elements:
+            return
 
         auto_satin(elements, self.options.preserve_order, starting_point, ending_point, self.options.trim)