Fix: handle case where startPt and stopPt resolve to same poin on line

lib/src/line_slice.dart

@@ -6,6 +6,9 @@ import 'package:turf/src/invariant.dart';
 /// and returns a subsection of the line in-between those points.
 /// The start & stop points don't need to fall exactly on the line.
 ///
+/// If [startPt] and [stopPt] resolve to the same point on [line], null is returned
+/// as the resolved line would only contain one point which isn't supported by LineString.
+///
 /// This can be useful for extracting only the part of a route between waypoints.
 Feature<LineString> lineSlice(
     Point startPt, Point stopPt, Feature<LineString> line) {
@@ -16,6 +19,10 @@ Feature<LineString> lineSlice(
 
   final startVertex = nearestPointOnLine(coords, startPt);
   final stopVertex = nearestPointOnLine(coords, stopPt);
+  if (startVertex['index'] == stopVertex['index']) {
+    // LineString do not allow 1 point lines
+    return null;
+  }
   late final List<Feature<Point>> ends;
   if (startVertex.properties!['index'] <= stopVertex.properties!['index']) {
     ends = [startVertex, stopVertex];