tweak default value

@@ -42,7 +42,7 @@ class ModalBottomSheet extends StatefulWidget {
   /// Creates a bottom sheet.
   const ModalBottomSheet({
     Key key,
-    this.closeProgressThreshold = _closeProgressThreshold,
+    this.closeProgressThreshold,
     this.animationController,
     this.animationCurve,
     this.enableDrag = true,
@@ -157,7 +157,8 @@ class _ModalBottomSheetState extends State<ModalBottomSheet>
       widget.animationController.value < _willPopThreshold;
 
   bool get hasReachedCloseThreshold =>
-      widget.animationController.value < widget.closeProgressThreshold;
+      widget.animationController.value <
+      (widget.closeProgressThreshold ?? _closeProgressThreshold);
 
   void _close() {
     isDragging = false;