Specify the program in launch config

.vscode/launch.json

@@ -8,7 +8,8 @@
             "name": "Flutter",
             "request": "launch",
             "type": "dart",
-            "flutterMode": "debug"
+            "flutterMode": "debug",
+            "program": "lib/main.dart"
         }
     ]
 }