fix null check

flutter/lib/pages/drawing_page.dart

@@ -101,7 +101,15 @@ class _DrawingPageState extends State<DrawingPage> {
           ),
           actions: [
             IconButton(
-              onPressed: () => graphData.flushData(),
+              onPressed: () {
+                setState(() {
+                  startDot = null;
+                  endDot = null;
+                  bfsPath = null;
+                  dfsAccessTable = null;
+                  graphData.flushData();
+                });
+              },
               icon: const Icon(Icons.delete_sweep),
               iconSize: 60,
             ),
@@ -126,6 +134,7 @@ class _DrawingPageState extends State<DrawingPage> {
                   dfsAccessTable = null;
                   startDot = null;
                   endDot = null;
+                  clearInputData();
                 });
               }),
               createButton(graphData.getUseLengthStr(), changeLength),