fix negative slice spans, error handling in promise chain, rebuild cleanup

src/geo/slicer.js

@@ -106,11 +106,6 @@
             }
         }
 
-        /** short-circuit for microscopic and invalid objects */
-        if (zMax == 0 || zSum == 0 || points.length == 0) {
-            return [];
-        }
-
         /**
          * bucket polygons into z-bounded groups (inside or crossing)
          * to reduce the search space in complex models
@@ -130,6 +125,11 @@
             });
         }
 
+        /** short-circuit for microscopic and invalid objects */
+        if (zSpan == 0 || zSum == 0 || points.length == 0) {
+            return {};
+        }
+
         // create empty buckets
         for (i = 0; i < bucketCount + 1; i++) {
             buckets.push({ points: [], slices: [] });